Acceleration sensor element and method of its manufacture

ABSTRACT

A flexure transducer element used in an acceleration sensor for sensing an acceleration applied thereto and the method of making the same. The flexure transducer element comprises a frame, a sheet member, a weight, and a support member.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a flexure transducer element and amethod of producing the same which is used for a semiconductoracceleration sensor having a both end supported beam structure and usedfor an automobile, an aircraft or a domestic electric appliance, andalso relates to an acceleration sensor including such an element. Forexample, such a sensor can be used for sensing an acceleration byseparately obtaining an X-axis component, Y-axis component and Z-axiscomponent of the acceleration applied thereto with respect to an X-Y-Zcoordinate rectangular system having the three axes.

2. Description of Background Information

The acceleration sensor as described above is disclosed in U.S. Pat. No.5,485,749. The sensor is a piezoresistor-type acceleration sensor whichconverts a mechanical flexure (or a strain) of a member generated by anacceleration into an electric signal, and one example of such a sensoris shown in FIGS. 18 (a schematic perspective view) and 19 (a crosssectional view taken along a line A-A′ in FIG. 18).

The acceleration sensor 500 includes a flexure transducer element 502and a bottom cover 504. The flexure transducer element 502 includes aframe 506 and a sheet member 508. The frame 506 has an upper surface 510and a lower surface 512 which is supported by a support member 514. Thesheet member 508 includes flexible parts 515 and a center part 516 (aportion surrounded by the dash and dot lines in FIG. 18). The flexiblepart 515 extends outward from the center part 516 and integrallyconnects with an inner edge 518 of the frame 502. A weight 520 connectswith the center part 516 of the sheet member 508 below the center part516.

An inward side surface 524 of the support member 514 is facing to anoutward side surface 526 of the weight 520 through a first space 528.Further, a second space 530 is present between the flexible parts 515and the weight 520, and connects with the first space 528. In addition,there is a third space 532 which is surrounded by the frame 506 and theflexible parts 515. The flexible parts 515 include a plurality ofpiezoresistors 534 and wirings (not shown) connected thereto on theirsurfaces.

The bottom cover 504 includes a periphery part 541 which defines arecess part 540 corresponding to and surrounding the weight 520, and thesupport member 514 is bonded to the periphery part of the bottom cover504 by an appropriate means such as anodic bonding. The bottom cover 504functions as a stopper which prevents the sheet member 508 from beingbroken due to over-displacement of the weight when an excessiveacceleration is applied.

When the acceleration sensor 500 as described above includes theplurality of the piezoresistors 534, it can be used as the accelerationsensor which detects the acceleration by obtaining separately the X, Yand Z axis components of the acceleration applied to the sensor withrespect to the X-Y-Z three axis regular coordinate defined by the X, Yand Z axes which regularly intersecting with one another (the X axis andthe Y axis extend over the upper surface defined by the sheet member 508and the frame 506).

Interconnections between the frame 506 and the sheet member 508 as wellas between the sheet member 508 and the weight 520 are such that whenthe acceleration is applied to the sensor 500, concretely to the element502, at least a portion of the flexible part 515 which portion has thepiezoresistor 534 is elastically deformed by the displacement of theweight 520 relative to the frame 506 (it is noted that the center part516 of the sheet member 508 which is connected to the neck part 522 issubstantially not deformed), and thereby a resistance change of thepiezoresistor 534 is converted to an electric signal. By detecting thesignal, the acceleration applied to the sensor is determined.

The production of the acceleration sensor as described above can becarried out based on a method disclosed in the U.S. Pat. No. 5,485,749,and concretely carried out as follows using a production sequence asshown in FIG. 20 which shows schematic cross sectional views similar toFIG. 19:

FIG. 20(a): First, a silicone nitride films 602 and 604 are formed onthe both surfaces of a first silicon substrate 600 from which thesupport member 514 and the weight 520 are to be formed.

FIG. 20(b): Then, an opening 606 is formed by removing a portion of thesilicon nitride film 602 which corresponds to the second space 530, andan opening 608 is formed by removing a portion of the silicon nitridefilm 604 which corresponds to the first space 528.

FIG. 20(c): By digging from the openings 606 and 608 to form recessparts 610 and 612 respectively, and then remaining silicon film 602 isremoved so that one surface of the first silicon substrate 600 isexposed, on which a second silicon substrate 616 is laminated so that aportion of the recess part 610 is formed into the second space 530 andthe rest part is formed into the neck part 522 of the weight and theupper surface of the support member 514.

FIG. 20(d): In order that the flexible part 515 is deformed upon theapplication of the predetermined acceleration when finally completed asthe sensor, the second silicon substrate 616 is thinned to a thickness(t) by grinding or etching, whereby the second silicon substrate isformed into the sheet member 508 and the frame 506.

FIG. 20(e): Then, the piezoresistors 618 are formed on the sheet member508 of the thinned second silicon substrate 616 using diffusion of animpurity of which conductivity type is different from that of the secondsilicon substrate 616.

FIG. 20(f): Then, after wirings (not shown) connected to thepiezoresistors 618 are formed, a first space 528 reaching the thirdspace 530 is formed by anisotropic etching from the recess part 612 sothat the weight 520 is connected to and supported integrally by thecenter part 516 of the second silicon substrate 616 through the neckpart 522.

Finally, the predetermined portion of the second silicon substrate 616is etched so that the third space 532 (not shown) is formed, whereby theflexure transducer element 502 is obtained. It is noted that the siliconnitride film 604 on the bottom surface of the first silicon substratemay be optionally removed.

The element 502 thus obtained is bonded to a bottom cover 504 (not shownin FIG. 20), which results in the piezoresistor-type accelerationsensor.

Alternatively, the following method is also known: the second space 530is not formed directly from the substrate, but a portion whichcorresponds to the second space is once formed as a sacrificial layer ofa polysilicon, and then the sacrificial layer is removed by supplying anetchant through the first space 528 after the first space 528 has beenformed (see Japanese Patent Kokai Publication No. 7-234242 and itscounterpart foreign patent applications if any and U.S. Pat. No.5,395,802).

In such an acceleration sensor, the acceleration to be detected isconverted to a flexure of the flexible part as at least a portion of thesheet member, so that the resistance of the piezoresitor formed on theflexible part is changed by means of the flexure, whereby finally theacceleration is converted to the electric signal.

Therefore, the sensitivity of the semiconductor acceleration sensor iscontrolled by particularly the thickness of the flexible part of thesheet member which is elastically deformed (or flexed). That is, whenthe flexible part becomes thicker, the sensitivity becomes worse, andthe sensitivity is affected by scattering of the thickness of theflexible part. Thus, the uniform and precise control of the thickness ofthe sheet member is important in the production process of thesemiconductor acceleration sensor.

As another type of the sensor, an electrostatic capacitance-type sensoris also known, and it is disclosed in for example Japanese Patent KokaiPublication No. 5-26754 and its counterpart foreign patent applications(if any) and Europe Patent Publication No. 0 461 265. Operationmechanism of such a sensor is similar to the piezoresistor-type sensorin that it is based on the mechanical flexure formed by the applicationof the acceleration. However, it is different from thepiezoresistor-type sensor in that the flexure is converted to relativedisplacement between two opposing members, and the displacement changesthe electrostatic capacitance between electrodes provided on themembers, which is utilized in the electrostatic capacitance-type sensor.Thus, in the electrostatic capacitance-type sensor, the electrodes areprovided on the member which is displaced and the member which is notdisplaced upon the application of the acceleration sensor so that theseelectrodes are opposing to each other.

Such an acceleration sensor is shown in FIGS. 21 (a schematic partiallycut-away perspective view) and 22 (a schematic cross sectional viewtaken along a diagonal C-C′ in FIG. 21). While the above flexuretransducer element 502 includes the piezoresistors 534, the flexuretransducer 702 of the acceleration sensor 700 includes in place of thepiezoresistors, the electrode 734 on the upper surface of the weight 520and the wiring 736 connected thereto, and the wiring is provided on thesheet member through the depressed corner 738 of the third space, Theother features are substantially the same as those of the abovepiezoresistor-type flexure transducer element 502 shown in FIGS. 18 and19.

It is noted that the flexure element 702 of the electrostaticcapacitance-type is used with the top cover 740 (not shown in FIG. 21)which is located on the element. The top cover 740 prevents excessivedisplacement of the weight, whereby prevents break of the flexibleparts, and includes on its inside, a recess part which corresponds to atleast the sheet member and preferably an upper surface of the elementexcept the frame. This kind of top cover is combined with the elementfor the piezoresitor-type acceleration sensor or the electrostaticcapacitance-type acceleration sensor, provided that in the latter typesensor, the top cover includes an electrode as described below. The topcover 740 includes the electrode 742 which faces to the electrode 734when the cover is placed on the element 702. In such an accelerationsensor, when the acceleration to be detected is applied to the sensor,the weight 520 is displaced relatively to the support member 514 andthus the cover 740 arranged thereon since the weight 520 is connected tothe sheet member 508 including the flexible parts 515. As a result, adistance between the electrode 734 on the weight and the electrode 742opposing thereto on the cover is changed, whereby the acceleration canbe sensed using an electrostatic capacitance change between theelectrodes which is caused by the distance change.

Also in this acceleration sensor of the electrostatic capacitance-type,when the thickness of the flexible part 515 is thinner, and also whenthe length of the flexible part is longer in the case of the flexiblepart being in the elongated form, the flexible part is more likely to bedeformed even with a smaller acceleration, which improves thesensitivity of the acceleration sensor. Also, when the thickness of theflexible part has scattering, scattering of the sensitivity occurs.

Therefore, in any type of the acceleration sensor, it is desirable thatthe thickness of the flexible part is properly controlled so that thesemiconductor acceleration sensor or the flexure transducer element isprovided which includes the flexible part having less scattering intheir thickness. Thus, it is important to precisely control the uniformthickness of the flexible parts in the production method of thetransducer element. Further, when the flexible part is in the elongatedform, it is preferable that its length can be longer.

In the production method of the prior art for the semiconductoracceleration sensor as described above provides such a sensor having aboth end supported beam structure in which the weight is connected tothe center part of the sheet member, the flexible parts of the sheetmember are connected to the frame, and the frame is supported by thesupport member.

In this production method, since the thickness scattering of the secondsilicon substrate is large in the step of thinning the second siliconsubstrate up to the predetermined sheet form thickness (t) afterlaminating the second silicon substrate 616 onto the first siliconsubstrate 600, it is difficult to control the thickness of the flexiblepart 515 uniformly. Further, lamination of the silicon substrates iscomplicated and two pieces of the silicon substrates are required, whichincreases the production cost.

SUMMARY OF THE INVENTION

The present invention is based on the above consideration as to theproblems as described above, and objects of the present invention are toovercome the above problems and thereby to provide a flexure transducerelement used for the semiconductor acceleration sensor and also aproduction method of the element, which includes a both end supportedbeam structure in which the sheet member, especially its flexible partthickness is formed with precisely controlling its thickness, and alsoto provide an acceleration sensor using such an element. Further, thepresent invention provides preferred embodiments of such an element, itsproduction method and such a sensor, and advantages achieved by thepresent invention will be clarified with reference to descriptions belowand the accompanied drawings.

The inventors have intensively studied the structure of thesemiconductor sensor and the production method thereof under theconsideration of the above problems, and found that when the sheetmember and the frame are formed of an epitaxial layer, the aboveproblems are overcome.

Thus, in the first aspect, the present invention provides a flexuretransducer element which is used in an acceleration sensor for sensingan acceleration applied thereto comprises

(1) a frame having an upper surface and a lower surface,

(2) a sheet member which has a plurality of flexible parts and a centerpart, each flexible part extending between at least a portion of aninner edge of the frame and the center part and being integrallyconnected to them,

(3) a weight which has a neck part integrally connected to the centerpart of the sheet member and which is hung from the sheet member throughthe neck part, and

(4) a support member which supports the lower surface of the frame andof which inward side surface faces to a side surface of the weightthrough a first space therebetween,

a second space which is continuous with the first space is definedbetween each flexible part of the sheet member and the weight,

a third space is defined between the frame and the sheet member and/orthrough the sheet member,

the frame and the sheet member are connected to each other and the sheetmember and the weight are connected to each other in such a manner that,when the acceleration is applied to the element, at least two flexibleparts are elastically deformed so that the weight is displacedrelatively to the frame,

the weight and the support member are formed of a semiconductorsubstrate,

the second space is formed by removing a sacrificial layer (or asacrifice layer) which is provided in the semiconductor substrate, and

the frame and the sheet member comprises an epitaxial layer provided onthe semiconductor substrate.

For example, the element can be used for an acceleration sensor such asa piesoresistor-type or electrostatic capacitance-type accelerationsensor which senses the acceleration by separately an X-axis, a Y-axisand a Z-axis components of the acceleration applied to the sensor withrespect to an X-Y-Z three coordinate system which is defined by thethree axes which intersect with one another. In this case, the X-axisand the Y-axis are so defined t hat they extend on an upper surface ofthe sheet member.

In the present invention, the flexure transducer element is intended tomean an element which converts the flexure in the acceleration sensor(such as the piesoresistor-type or electrostatic capacitance-typeacceleration sensor as described above) into an electric output.

It is noted that in principle, the same terms are used for members andparts for the present device as in used for the members and the partsfor the prior art device described above in descriptions as to thepresent invention.

In the element according to the present invention, the weight and thesupport member are made of a single semiconductor substrate, and theframe and the sheet member are made of the epitaxial layer grown on thesemiconductor substrate. The weight, the frame, the sheet member and thesupport member are so connected that they constitute a structure inwhich at least a portion of the flexible part of the sheet member iselastically deformed (or flexed) when the acceleration is applied to theelement,

The sheet member is formed of the epitaxial layer as described above inthe present invention, the element having the flexible parts each havinga more uniform thickness is provided when compared with the elementproduced by the prior art method in which the silicon substrate islaminated followed by mechanically reducing its thickness.

In one preferable embodiment of the flexure transducer element accordingto the present invention, the first space and the second space defineside surfaces of the weight, the weight has such a structure that it isconnected to the center part of the sheet member through the narrow neckpart. That is, when considering a cross section of the weight which isparallel to the semiconductor substrate, the neck area is smaller thanthe other portion of the weight and the cross section of the neck partis located in the center of the other portion.

The form of the weight is not particularly limited. For example, whenthe element has an overall form of a square prism, the weight may besubstantially in the form of the square prism except the neck part. Theneck part may be a square prism (or a column) form having a small heightwhich is included by and concentric with the square (or quadratic)prism. In order to make the volume of the weight as much as possiblerelatively to the total volume of the element, the neck part ispreferably as small as possible, and the cross sectional area of theweight is preferably as large as possible. It is of course that theweight does not have to be larger when the small weight is sufficient.It is noted that the weight may be made of only the semiconductorsubstrate or made of the semiconductor substrate and a portion of theepitaxical layer which is formed thereon.

In one preferable embodiment of the flexure transducer element accordingto the present invention, the flexible parts of the sheet member includeat least two portions each being able to elastically deform upon theapplication of the acceleration and having at least one piezoresistor,and each of the piezoresistors has a wiring connected thereto. Thewiring may be any wiring which can send information or output related toan electric signal converted from resistance change of thepieozoresistor. For example, the wiring may be a metal wiring and/or adiffusion wiring. Further, when the wiring is the metal wiring, it maybe connected directly to an electrode pad, or when the wiring is thediffusion wiring, it may be connected to the electrode pad through themetal wiring. Through the electrode pad, the element is connected to anapparatus in which the element measures a piezoresistance.

The position where the piezoresistor is located is not particularlylimited provided that the flexure of the flexible part can beelectrically detected. In fact, there are various arrangements of thepiezoresistors, but it is preferable that it is located at a position ofthe flexible part where the elastic flexure (or deformation) isconcentrated. As to the concrete arrangement of the piezoresistors aredisclosed in U.S. Pat. No. 5,485,749 and Japanese Patent KokaiPublication Nos. 6-331646, 6-109755 and 7-234242 and their counterpartforeign patent applications (if any), the disclosures thereof arereferred to for the concrete arrangement of the piezoresistors in thepresent invention. It is noted that the disclosures are incorporatedherein by these references.

Also, it is noted that such an element is connected with the bottomcover and the top cover as described above, which results in theacceleration sensor. Therefore, the present invention provides thepiezoresistor type acceleration sensor which includes the element asdescribed above and the bottom cover and the top cover. As describedabove, the bottom cover and the top cover have the recess parts on theirinward sides, and when an excessive acceleration is applied to theacceleration sensor, they prevent the sensor, particularly the flexibleparts from being broken.

In one preferable embodiment, the flexure transducer element accordingto the present invention includes at least one electrode for theelectrostatic capacitance measurement on at least one portion (forexample, a portion of the sheet member or the weight upper surface)which is displaced by the elastic deformation of the flexible parts uponthe application of the acceleration. The electrode includes a wiringconnected thereto. The wiring may be any wiring which can send outputrelated to the electrostatic capacitance measurement. For example, thewiring may be a metal wiring. Further, when the wiring is the metalwiring, it may be connected directly to an electrode pad, or when thewiring is the diffusion wiring, it may be connected to the electrode padthrough the metal wiring. Through the electrode pad, the element isconnected to an apparatus in which the element measures theelectrostatic capacitance.

The position where the electrode for the electrostatic capacitancemeasurement is located is not particularly limited provided that itconstitutes an electrode which is relatively displaced to an electrodeof the top cover provided above the element while the electrode isopposing to said electrode of the top cover. Various arrangements may bepossible as to the electrode, but the electrode is preferably providedon a portion of which displacement due to the deformation of theflexible parts is large. For example, the electrode may be provided on aportion of the upper surface of the weight near its outer periphery asshown in FIG. 21 (for example, the electrode 734). More concretearrangements as to the electrode for the electrostatic capacitancemeasurement are disclosed in Japanese Patent Kokai Publication No.5-26754 and its counterpart foreign patent applications (if any) andEurope Patent Publication (A1) No. 0 461 265, and the disclosuresthereof are referred to for the concrete arrangement of the electrodefor the electrostatic capacitance measurement in the present invention.It is noted that the disclosures are incorporated herein by thesereferences.

Such an element is connected with the optional bottom cover and the topcover as described above, which results in the acceleration sensor.Therefore, the present invention provides the electrostaticcapacitance-type acceleration sensor which includes the element asdescribed above and the bottom cover and the top cover. As describedabove, the bottom cover and the top cover have the recess parts on theirinward sides, and when an excessive acceleration is applied to theacceleration sensor, they prevent the sensor, particularly the flexibleparts from being broken. It is noted that the top cover includes theelectrode which faces to the electrode provided on the element.

The element as described above according to the present invention isproduce by the following method. Therefore, the present inventionprovides a method for the production of the flexure transducer elementwhich is used for the acceleration sensor according to the presentinvention as described above and below, the method comprising the stepsof:

(1) forming in the first main surface of the semiconductor substrate forthe formation of the weight having the neck part and the support member,the sacrificial layer which extends outward from a portion of an outerperiphery of the center part of the first main surface which center partis to constitute the neck part,

(2) the epitaxial layer is formed on the first main surface after step(1), and

(3) after step (2), carrying out the following sub-steps (3-a) to (3-c):

(3-a) removing a portion of the substrate from the second main surfaceof the substrate using etching so that the side surface of the weightand the support member are formed, the support member including the sidesurface opposing to the side surface of the weight through the firstspace,

(3-b) forming the third space through the epitaxial layer by removing aportion thereof using etching so that at least a portion (optionallysubstantially all) of the rest of the eptaxial layer is formed into theframe and the sheet member including the center part and a plurality ofthe flexible parts which finally becomes able to elastically deform, and

(3-c) removing the sacrificial layer through wet etching so that thesecond space and the neck part of the weight are formed, whereby theweight is formed, in any one of the following sub-step orders (i) to(iv):

(i) sub-step (3-a)→sub-step (3-b)→sub-step (3-c),

(ii) sub-step (3-a)→sub-step (3-c)→sub-step (3-b),

(iii) sub-step (3-b)→sub-step (3-a)→sub-step (3-c), and

(iv) sub-step (3-b)→sub-step (3-c)→sub-step (3-a).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a partially cut-away perspective view of anelement for a piezoresitor-type acceleration sensor according to thepresent invention;

FIG. 2 shows a top view of the element of FIG. 1;

FIGS. 3(a) to (i) show in cross sectional views, a series of productionsteps of an element according to the present invention;

FIGS. 4(a) to(c) show in schematic partially cut-away perspective views,production steps of the element of FIG. 3;

FIGS. 5(a) to (l) schematically show top views so as to show shapes andarrangements of an etchant introduction ports;

FIGS. 6(a) and (b) show schematic partially cut-away perspective viewsof other embodiment of an element according to the present invention;

FIGS. 7(a) to (i) show in cross sectional views, a series of productionsteps of another element according to the present invention;

FIGS. 8(a) to (e) show in schematic partially cut-away perspectiveviews, production steps of the element of FIG. 7;

FIG. 9 shows a schematic cross sectional view of another embodiment of afirst space;

FIGS. 10(a) to (h) show in cross sectional views, a series of productionsteps of an element having the first space shown in FIG. 9;

FIGS. 11(a) to (h) show schematic cross sectional views of theproduction method for an element according to the present inventionincluding steps for the formation of a wiring protection layer;

FIGS. 12(a) to (e) show schematic cross sectional views of theproduction method for an element according to the present inventionincluding other steps for the formation of a wiring protection layer;

FIGS. 13(a) to (d) show schematic cross sectional views of theproduction method for an element according to the present inventionincluding further other steps for the formation of a wiring protectionlayer;

FIGS. 14(a) to (c) shows schematic cross sectional views of one exampleof formation steps of a sacrificial layer having a small impurityconcentration in a surface portion thereof;

FIGS. 15(a) to (d) shows schematic cross sectional views of anotherexample of the formation steps of a sacrificial layer having a smallimpurity concentration in a surface portion thereof;

FIGS. 16(a) to (e) shows schematic cross sectional views of a furtherexample of the formation steps of a sacrificial layer having a smallimpurity concentration in a surface portion thereof;

FIG. 17 schematically shows a perspective view of an apparatus for theproduction of a porous silicon layer as a sacrificial layer;

FIG. 18 shows a schematic cross sectional view of a piezoresistor-typeacceleration sensor which has been known from the prior art;

FIG. 19 shows a schematic cross sectional view of the accelerationsensor of FIG. 18;

FIGS. 20(a) to (f) show in cross sectional views, a series of productionsteps of the acceleration sensor of FIG. 18;

FIG. 21 shows a schematic partially cut-away cross sectional view of anelectrostatic capacitance-type acceleration sensor which has been knownfrom the prior art; and

FIG. 22 shows a schematic cross sectional view of the accelerationsensor of FIG. 21.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The semiconductor substrate used in the method according to the presentinvention may be a silicon substrate, and its conductivity type may beeither a P-type or an N-type. The weight and the support member are madeof this substrate. As the semiconductor substrate, for example an N-typesubstrate may be used of which orientation of a crystal plane (crystalsurface indices) is (100). An impurity concentration of the substrate isdesirably not more than 1.0×10¹⁷ cm⁻³ (for example, in the range between1.0×10¹⁴ cm⁻³ and 1.0×10¹⁶ cm⁻³). When the substrate having such animpurity concentration is used, an etching speed is reduced to about1/150 or less of an etching speed when using a substrate having a largerimpurity concentration, so that a layer having such a smaller impurityconcentration is left as a low impurity concentration layer even when alayer having a larger impurity concentration is removed by etching.Further, a thickness of the substrate is not particularly limited, andit may be selected appropriately depending on an application of thesensor. Generally, the thickness of the substrate may be the same as ora little thicker than that of the conventionally used substrate for theacceleration sensor. For example, the substrate having a thickness of400 μm to 600 μm may be used. On one of main surfaces of such asubstrate is formed the sacrificial layer. The term “sacrificial layer”is used to mean a layer which is present during the production method ofthe element according to the present invention, but finally removed toprovide a space.

The sacrificial layer extends outward from a portion corresponding to acenter portion of the semiconductor substrate. The center portion is aportion which is to become the neck part of the weight and finallyconnected to the center part of the sheet member, and no sacrificiallayer is formed in the center portion. A shape of the center portion ofthe semiconductor substrate is not particularly limited, and for exampleit may be a circle or a rectangle (square, oblong). Particularly, it ispreferable that the center of gravity of the weight is below the centerportion, especially below the central point of the center portion of thesubstrate. The sacrificial layer extends outwardly from an outerperiphery of the center portion of the substrate. The sacrificial layermay extend from an entire of the outer periphery of the center portion(i.e. a whole periphery of the center portion so as to enclose thecenter portion) so as to enclose the center portion, or from a portionof the outer periphery of the center portion.

When the sacrificial layer extends from the entire of the outerperiphery, it may be an annular shape. For example, the center portionof the substrate is of a circular shape, and the sacrificial layer isformed by a circle which is concentric with that circular shape of thecenter portion, and thus the sacrificial layer may be an annular portionbetween the concentric circle and the center portion. In otherembodiment, the center portion is of an inner square, and thesacrificial layer is formed by another outer square which is concentricwith that inner square, and thus the sacrificial layer may be an annularportion between the inner square and the outer square. The sacrificiallayer may be a portion formed by combination a circular center portionwith an outer square or vice versa. In place of the square, arectangular and in place of the circle, an oval may be used.

When the sacrificial layer extends from a portion of the outer peripheryof the center portion, the sacrificial layer may be substantiallyelongated layers which extend at an evenly spaced angle (for example90°) from the periphery of the center portion. In case where the angleis 90°, the sacrificial layer is in the form of four beams which areopposing through the center portion with each other (i.e. thecross-shape having an intersection point in its center). In other words,the sacrificial layers may extend radically from the center portion, andthe number of the sacrificial layers is not limited, but usually four isenough. Further, in other embodiment, the elongated sacrificial layerspreferably extend symmetrically (with respect to a point or an axis)from the center portion of the substrate.

It is noted that a thickness of the sacrificial layer substantiallycorresponds to a distance between the flexible part and an upper surfaceof the weight (thus, a thickness of the second space), and therefore isselected depending on an application of the sensor. For example, thethickness may be for example 5 to 15 μm.

In the method according to the present invention, the sacrificial layercontains the impurity of which conductivity type is the same as oropposite to that of the impurity of the substrate body, and may beprovided by forming a portion of which impurity concentration is largerthan that of the substrate (i.e. forming the high impurity concentrationlayer) in the surface of the substrate, or forming a porous siliconlayer in the surface of the substrate.

Those skilled in the art may easily select the impurity concentration ofthe high impurity concentration layer based on etching conditions, anetching path and so on while considering the impurity concentration ofthe substrate body. For example, when the impurity concentration of thesubstrate body is in the range between about 1.0×10¹⁴ cm⁻³ and about1.0×10¹⁶ cm⁻³, the impurity concentration of the high impurityconcentration layer may be in the range between about 1.0×10¹⁸ cm⁻³ andabout 1.0×10²⁰ cm⁻³ (or a solid solubility).

It is known that when the sacrificial layer is removed by etching, theimpurity concentration in the sacrificial layer is increased so as toimprove an etching selectivity ratio (i.e. a ratio of an etching speedas to a certain material to another etching speed as to another certainmaterial). (For example, see B. Schwarts, “Chemical Etching of Silicon”,SOLID-STATE SCIENCE AND TECHNOLOGY, pp 1903-1909, December 1976.)

The porous silicon layer as the sacrificial layer may be provided byforming a silicon oxide film on a silicon substrate, then formingthrough the silicon oxide film, an opening corresponding to thesacrificial layer, and then carrying out the deposition and the thermaldiffusion or the ion implantation and the annealing treatment of forexample a P-type impurity through the opening followed by anodicoxidation in an electrolyte solution (for example a solution containinghydrofluoric acid).

The formation of the sacrificial layer having a predetermined shape maybe carried out by the ion implantation and the annealing treatment orthe deposition and the thermal diffusion after masking with aphotoresist. The thickness and the impurity concentration of thesacrificial layer may be controlled by appropriately selecting operationconditions upon the formation. These technical knowledge is well-knownto those skilled in the art.

Then, the epitaxial layer is formed on an entire surface on the side ofthe substrate which side has the sacrificial layer in step (2). Sincethe epitaxial layer finally constitutes the sheet member of the element,its thickness has to be such that the sheet member is able toelastically deform so that the acceleration is detected with apredetermined sensitivity. When the thickness is thinner, the smalleracceleration can be detected since deformation is possible even by asmall acceleration, but the sheet member is likely to be broken, andvice versa. Therefore, depending on a predetermined application of theelement, the thickness has to be selected. The formation method of theepitaxial layer is well-know to those skilled in the art. As to theformation of the eptaxial layer on the porous silicon layer, JapanesePatent Kokai Publication No. 5-217990 and its counterpart foreign patentapplications (if any) may be referred to, and the disclosures thereinare incorporated herein by these references.

In step (3), various etching is carried out so as to form the sheetmember, the support member, the weight, the first space, the secondspace and the third space. As to the order of sub-steps (3-a) to (3-c)in step (3), there is no limitation in the order of the sup-stepsprovided that sub-step (3-c) is not carried out at first.

In sub-step (3-a), the substrate is etched so that the support member,the side surface of the weight and the first space between them areformed. Etching is carried out from the side of the second main surface(i.e. the surface which does not include the eptaxial layer) of thesubstrate so that a portion of the substrate is removed. This etching isso carried out that the first space is present around the weight and thesupport member surrounds the first space. In one preferable embodiment,the substrate is a sheet in the form of a square, the support member isa wall member which is located on four edge periphery of the square andsurrounds the square, and within the wall member, the weight having asquare cross section (parallel to the main surface of the substrate) islocated so that the first space is present between the wall member andthe weight. The cross section of the weight is not necessarily thesquare, and it may be for example, circular, rectangular and so on.However, from a viewpoint that the weight is to be as large as possible,the cross section of the weight is preferably the square when thesubstrate has a square shape. When the substrate is rectangular, theweight preferably has a rectangular shape in its cross section which issimilar to the rectangle of the substrate.

Sub-step (3-b) forms the third space in the epitaxial layer as anthrough opening and thereby the flexible parts are so formed that theyare finally able to elastically deform, and also forms the frame. Thisis based on that by forming the eptaxial layer into the sheet form whichincludes the through opening so that the epitaxial layer partiallyincludes the elongated parts rather than the epitaxial layer being flatand wide, the epitaxial layer is likely to elastically deform. In oneembodiment, the eptaxial layer which is left by the formation of thethird space in this sub-step constitutes in addition to the frame andthe sheet member, the weight upper part (part 41 of FIG. 1 or part 91 ofFIG. 6(a)). It is noted that no sacrificial layer is present in thesubstrate which is located below the weight upper part. Therefore, theweight upper part and the substrate remain integral together all thetime. Even after sub-step (3-b) is completed, the flexible parts cannotbe elastically deformed when the sacrificial layer is present below theflexible parts, and become able to elastically deform first after thesacrificial layer is removed. In this meaning or in the meaning “uponbeing completed as the element”, the term “finally” is used.

Sub-step (3-c) removes the sacrificial layer through etching so that thesecond space and the neck part of the weight are formed. When the firstspace and/or the third space are not formed, the second space cannot beformed. Therefore, this sub-step cannot be carried out first.

Those three kinds of spaces forms a single space together.

Etching is used in the sub-steps of step (3). Depending on the shape andsize of the space to be formed in any of the sub-steps, anisotropicetching (including the reactive ion etching (RIE)) or isotropic etchingis used. In principle, the anisotropic etching is used for the formationof the first space and the third space, and the isotropic etching isused for the second space. These etching methods are well-known to thoseskilled in the art, and in order to carry the method according to thepresent invention, the methods disclosed in for example Japanese PatentKokai Publication Nos. 2-81477 and 5-340957 and their counterpartforeign patent applications (if any) and U.S. Pat. No. 4,882,933.

In the method according to the present invention, step (3) may comprisesub-step (3-d) of forming at least one piezoresistor on at least oneflexible part of the epitaxial layer. Before or after forming thepiezoresistor, a wiring may be formed which is connected to thepiezoresistor. Instead of sub-step (3-d), step (3) may comprise sub-step(3-e) of forming an electrode for the electrostatic capacitancemeasurement on a portion of the epitaxial layer which is displacedrelatively to the frame upon the application of the acceleration,especially on that portion which constitutes the weight (namely on theweight upper part). Before or after, or simultaneously with theformation of this electrode, a wiring which is connected thereto may befurther formed. When the weight does not include the epitaxial layer(namely, when the weight is made of only the substrate), the electrodemay be formed on the weight. In this case, the electrode is formed afterthe formation of the third space. Sub-step (3-d) or (3-e) may be carriedout in any stage during step (3) except the last embodiment. The wiringconnected to the piezoresistor is preferably a diffusion wiring. Theelectrode for the electrostatic capacitance measurement is preferably ametal wiring. Further, when etching is carried out after such apiezoresistor or electrode and optionally the wiring if any have beenformed, the epitaxial layer which includes the piezoresistor orelectrode and the optional wiring is preferably protected by aprotection film, for example a silicon oxide film and/or a siliconnitride film in order to avoid an effect of the etching which issubsequently carried out.

Thus, step (3) may comprise after sub-step (3-d) or (3d), sub-step(3-f-1) of providing the protection film which covers the piezoresistoror electrode and the optional wiring. The protection film may be atleast one film, and when it is formed as two layers by laminating them,there is an advantage that the flatness of the substrate is kept if thelayers are arranged while their bending directions are opposed to eachother.

In order that an electric signal is transferred to other member, forexample a signal processing apparatus from the piezoresistor or theelectrostatic capacitance measurement electrode directly or through awiring connected to the piezoresistor or the electrostatic electrostaticcapacitance measurement electrode, another wiring such as a metal wiringand a pad electrode connected thereto may be provided with the element.When such a wiring and a pad electrode are provided, the protectionlayer on the predetermined portion of the piezoresistor (or theelectrostatic electrostatic capacitance measurement electrode) or on thepredetermined portion of the wiring connected the piezoresistor (or theelectrostatic electrostatic capacitance measurement electrode) isremoved, and another wiring which is directly connected to thepiezoresistor (or the electrostatic electrostatic capacitancemeasurement electrode) is formed, or another wiring and the electrodepad are formed which are connected to the wiring connected to thepiezoresistor (or the electrostatic electrostatic capacitancemeasurement electrode). When etching is carried out after forming saidanother wiring and the electrode pad, a wiring protection layer ispreferably formed which protects said another wiring and the electrodepad, so that an effect on them of the etching can be avoided. Thus, whensaid another wiring and the electrode pad are formed after sub-step(3-f-1) after sub-step (3-d) or (3-e), and then etching is subsequentlycarried out, sub-step (3-f-2) of forming the wiring protection layer maybe included which protects said another wiring and the electrode pad.

Thus, step (3) of the present method may comprises sub-step (3-f) ofupon the formation of the piezoresistor or the electrostatic capacitancemeasurement electrode, the wiring or the electrode pad, forming thewiring protection layer which protects them before etching if they areaffected by etching which is subsequently carried out.

In step (3) of the present method, the removal of the sacrificial layermay be carried out after the first space has been formed, or after thethird space has been formed , or after the first space and the thirdspace have been formed. When the first space or the third space has beenformed, an etchant which removes the sacrificial layer can be suppliedthrough the space. The introduction of the etchant may be carried outthrough the first space and/or the third space.

When the third space leading to the sacrificial layer is formed and theetchant is supplied through the space, the third space is preferablyformed in a portion through the epitaxial layer which portion is locatedabove the sacrificial layer to be removed (for example a portion of theepitaxial layer which portion is to be the flexible part) and/or anotherportion adjacent to said former portion. It is more preferable that thethird space is formed on all of the epitaxial layer located on thesacrificial layer to be removed excluding a portion to be the flexibleparts. The formation of the third space may be carried out in anyetching method depending on the shape of the space, and generallyanisotropic etching is used.

For example, when the sacrificial layer is elongated and that portion ofthe epitaxial layer which is above the sacrificial layer (thus, saidportion is also elongated similarly) is converted into the flexiblepart, the third space is formed in the epitaxial layer so that it islocated outside and adjacently to the flexible part to be formed, and atleast partially and preferably entirely along the periphery of theflexible part to be formed. When the third space is formed in this way,etching can be carried out along a direction perpendicular to thelongitudinal direction of the flexible part (namely, a width directionof the flexible part) from a position along the longitudinal directionof the flexible part, while when the second space is formed after theformation of the first space, etching has to be carried out from theperiphery of the weight toward its center portion. Thus, in the formercase, there is an advantage in that a path which etching has to followis shortened (therefore, a period for the removal with etching can bereduced).

Alternatively, the third space may be formed in a portion of theepitaxial layer which portion corresponds to the flexible part such thatthe space passes through the epitaxial layer. Also in this case, thethird space is preferably formed so that it extends along thelongitudinal direction of the flexible part based on the same reason asdescribed above.

When the sacrificial layer surrounds the center portion of the substrateand extends outward from the outer periphery of the center portion ofthe substrate, it is also preferable that the third space is formed byetching the epitaxial layer excluding portions of the epitaxial layer tobe left as the flexible parts and frame so that the sacrificial layer isexposed at the bottom of the third space and the sacrificial layer issubsequently removed using etching through the third space.

The third space may be formed by subjecting the eptaxial layer directlyto the anisotropic etching or the RIE, or forming a second high impurityconcentration layer in the eptaxial layer (it is noted that the firsthigh impurity concentration layer is the sacrificial layer formed in thesubstrate) followed by removing the second high impurity concentrationlayer using etching. The manner with which the third space is formeddepends on the shape and size of the third space to be formed. The thirdspace, particularly a portion which reaches (or connected with) thesacrificial layer becomes an etchant introduction port (or opening).Particularly, when the etchant introduction port is formed using theRIE, the flexible parts can be precisely formed.

In the case in which the third space is formed, the anisotropic etchingconditions are preferably so selected that the opening of the thirdspace in the epitaxial layer which opening is remote from the substrateautomatically stops the etching when the etching has proceeded up to thesacrificial layer. Such selection may be carried out by controlling thesize and shape of the opening of a mask upon the anisotropic etchingbased on the anisotropic etching properties.

In a preferred embodiment, a cross section along the epitaxial layer ofthe third space, namely the shape of the etchant introduction port iscircular, oval, rectangular (especially having four rounded corners) orany combination thereof. Particularly, there is an advantage thatmechanical strength against the stress concentration is improved withouta sharp corner.

In the method according to the present invention, when the second highimpurity concentration layer is formed and then removed by etching,etching of the first high impurity concentration layer as thesacrificial layer can be carried out subsequently to the etching of thesecond high impurity concentration layer, so that the productionsequence may be shortened.

In the present invention, the (first) high impurity concentration layercontaining the impurity of the second conductivity type is formed as thesacrificial layer on the first main surface of the semiconductor siliconsubstrate of the first conductivity type, an impurity concentration ofthe high impurity concentration layer is preferably lower in the surfacethan in the inside thereof (or an inner side from the surface of thehigh impurity concentration layer). That is, a concentration profile ofthe impurity along a thickness of the high impurity concentration layerhas a peak (maximum) at a certain position remote inward from thesurface. In this manner, upon the initiation of the eptaxial growth onthe substrate having the high impurity concentration layer, an amount ofthe impurity which escapes from the high impurity concentration layerinto a growth atmosphere can be lowered. As a result, formation of aninversion layer due to auto-doping as well as diffusion of the impurityinto the epitaxial layer to be formed are suppressed. In a preferableembodiment, the impurity concentration in the surface of the highimpurity concentration layer is not more than 5×10¹⁹ cm⁻³ and not lessthan 1.0×10¹⁷ cm⁻³.

Such a high impurity concentration layer may be formed by the depositionand the thermal diffusion of the impurity into the substrate followed bythe wet etching or the pyrogenic oxidation. Alternatively, the impurityconcentration in the surface may be lower relative to that of the insideof the impurity layer by the direct implantation of an impurity ion intothe substrate followed by annealing. In a further alternative, theimpurity layer is formed beforehand and then another impurity having aconductivity type opposite to that of the impurity of the impurity layeris doped near the surface of the impurity layer so that the impurityconcentration in the surface portion is made relatively smaller thanthat of the inner portion.

Upon the formation of the sacrificial layer on the substrate, when theimpurity concentration of the first conductivity type of at least theepitaxial layer selected from the epitaxial layer and the substrate islarger than the concentration of the second impurity for the impuritylayer which can be taken into the epitaxial layer by auto-doping uponthe epitaxial growth, the N-type impurity and the P-type impurity arecompensated with each other, whereby the inversion of the conductivitytype of the substrate is prevented.

In one preferable embodiment of the present invention, the cross sectionof the first space which is taken through the center of the substrateand perpendicular to the substrate is so tapered in two step along adirection from the weight bottom to the neck part (i.e. an upwarddirection with respect to the substrate) that a distance between thesupport member and the weight is decreased, so that the first space isconstituted by a first part near the weight bottom and a second part,and a tapering angle of the first part is smaller than that of thesecond part. That is, the gap between the inward side surface of thesupport member and the side surface of the weight becomes smaller whenbeing closer to the eptaxial layer. The element as described is producedupon the formation of the first space by forming the first part usingmechanically or chemically grinding, and then forming the second partusing the anisotropic etching.

As described above, in one preferable embodiment of the presentinvention, before the etching, for example before the etching to removethe sacrificial layer, the wiring protection layer is formed so as tocover the piezoresistors (or the electrode for the electrostaticcapacitance measurement) provided on the epitaxial layer, the wiringconnected thereto, and another wiring and the electrode pad if any, andthen the sacrificial layer is removed, and then that portion of thewiring protection layer which is on at least the electrode pad isremoved with etching so as to expose the electrode pad. Since thesacrificial layer is removed with etching after the wiring protectionlayer is formed, it is prevented that the piezoresistors (or theelectrode for the electrostatic capacitance measurement), the wiring andthe electrode pad are corroded or broken because of the etchant for theremoval of the sacrificial layer through etching, which improves a yieldas well as reliabilities of the chip.

The wiring protection layer may be a chromium film, silicon nitride filmor a fluoroplastic (including its composition). The wiring protectionlayer of the silicon nitride film may be formed by for example theplasma CVD method. When the silicon nitride film is used as theprotection layer, it is preferably formed at a low temperature such asnot higher than 300° C. since aluminum generally used for the wiring maycause an alloy spike problem at a temperature above 500° C.

When the fluroplastic is used as the wiring protection layer, it isconvenient in that the fluoroplastic does substantially not disappearupon the removal of the sacrificial layer. Concretely, the fluoroplasticresin such as CYTOP CTL-809M (a composition of a fluoroplastic(C₆F₁₀O)_(n) and tris(perfluorobutyl)amine from Asahi Chemical) may beused. The wiring protection layer may be formed by the sputtering or thevapor deposition in the case of the chromium film, and by dissolving theresin into a proper solvent followed by the spin coating in the case ofthe fluoroplastic resin.

When the wiring protection layer has been formed, it is possible topattern-etch only a portion of the wiring protection layer which is onthe electrode pad so as to thin only the portion by a desired thickness,and then the only electrode pad is exposed by etching the wiringprotection layer over its entire surface after the removal of thesacrificial layer with etching. In this case, the wiring protectionlayer covers all except the electrode pad, so that moisture resistanceof the sensor is improved. After the removal of the sacrificial layerwith etching, the wiring protection layer has irregularities on itssurface and substrate strength is reduced, so that carrying outpatterning (for example photolithography step) becomes difficult.However, only the portion of the wiring protection layer on theelectrode pad is thinned beforehand, etching over the entire surfaceafter the etching removal of the sacrificial layer exposes only theelectrode pad without patterning.

In the embodiment wherein the wiring protection layer is formed, thefollowing is possible: etching to form the first space is stopped beforeit reaches the sacrificial layer so that a thin portion of thesemiconductor substrate is left below the sacrificial layer, then anetchant introduction port as the third space which reaches thesacrificial layer is formed through the wiring protection layer and theepitaxial layer, then an etchant is supplied through the etchantintroduction port so that the sacrificial layer is removed, and thensaid thin portion is removed through etching. In this embodiment, uponthe removal of the sacrificial layer, the substrate is unlikely to bebroken. The removal of the thin substrate portion may be carried out byanisotropic etching using an alkaline based etchant or the RIE.

It is noted that it may be advantageous to etch the bottom of the weightso that the weight has a thinned thickness. This is because the bottomcover having a flat form (i.e. without the recess part) may be used.Such etching may be carried out simultaneously with the etching toremove the semiconductor substrate portion left below the sacrificiallayer.

Various embodiments of the present invention will be hereinafterexplained more concretely with reference to the accompanied drawings,which do not limit the present invention.

First, one embodiment of the method of the present invention will beexplained more concretely with reference to FIGS. 1 to 4.

One example of the flexure transducer element (for thepiezoresister-type acceleration sensor) of the present invention whichis produced by the method according to the present invention is shown ina partially cut-away perspective view in FIG. 1 and in a top view (whenseeing the element of FIG. 1 from the above) in FIG. 2.

The flexure transducer element of the present invention 10 comprises theframe 12 and the sheet member 14. The frame 12 includes the uppersurface 16 and the lower surface 18, and the lower surface 18 issupported by the support member 20. The sheet member 14 is substantiallycomposed of the flexible parts 15, the center part 22 and the weightupper part 41, and the flexible part 15 extends outward from the centerpart 22 and integrally connected to the inner edge 24 (shown with thebroken line in FIG. 1) of the frame 12. The center part 22 of the sheetmember 14 includes the weight body 26 below it, and the weight body 26is connected integrally to the center part 22 through the neck part 28(see FIG. 3(i)). In the shown embodiment, the weight body 26 includesthe weight upper part 41 thereon, and these together substantiallyconstitute the weight 26′ of the element.

The inward side surface 30 of the support member 20 opposes to theoutward side surface 34 of the weight 26 through the first space (orgap) 36. Further, the second space (or gap) 38 is located between theflexible parts 15 and the weight body 26, and the space is connected tothe first space. In addition, the space 39 is located between the weightupper part 41 and the flexible part 15, the space 43 is located betweenthe frame 12 and the weight upper part 41, and these spaces constitutethe third space (or gap) 40. The sheet member 14, particularly theflexible part 15 includes on its surface, a plurality of thepiezoresistors 42 (not shown in FIG. 2) and wires which are connected tothe piezoresistors (not shown). It is noted that the space 43 and thefirst space 36 are connected to each other so as to constitute a slitform, and the second space 38 is connected to the first space 36 and thethird space 40 and thus these spaces form a single space.

FIG. 3 shows a sequence of the production method of the element 10 shownin FIGS. 1 and 2 in cross sectional views taken along the line B-B′.

FIG. 3(a): First, a single crystal silicon substrate 50 is prepared asthe semiconductor substrate of which conductivity type is N. When theelement (having a size of for example 5 mm×5 mm) is produced, aplurality of the elements (for example 200-300 elements) are actuallyproduced which are integrally adjacent to one another (in such a mannerthat the elements are arranged one on the other and one next to theother when seeing the substrate from its above) using a circularsubstrate (having a diameter of for example four inches) followed bycutting and dividing them into each single element using a dicing saw,which is generally carried out in the art of the semiconductor elements.Although the element and its production method of the present inventionare explained with reference to the single element for ease ofunderstanding, it is obvious for those skilled in the art that suchexplanations are applicable to the production of the plurality of theelements. Therefore, the substrate may be in a generally rectangularform or a square form in the sense of the production of the singleelement.

FIG. 3(b): Next, four sacrificial layers 56 each in the form of asubstantially elongated rectangle are formed in the first main surface58 of the substrate 50 which layers extend from the four sides 52 of therectangular or square center part 23 of the silicon substrate 50 towardthe outer periphery of the substrate, but terminate at the positions 54before the periphery. The formation of the sacrificial layers is carriedout by under the consideration that the flexible parts 15 are formed onthe sacrificial layers, masking all of the first main surface exceptthose portions on which the sacrificial layers are to be formed and thenby ion implantation of a P-type impurity such as boron into thenon-masked portions at a high impurity concentration followed byannealing, so that those portions are formed of which P-type impurityconcentration is high. In this step, the sacrificial layer 56 preferablyhas a width which is a little wider than that of the flexible part 15.

FIG. 3(c): Then, the eptaxial layer 60 of which conductivity type is Nis formed on the whole of the main surface 58 of the substrate 50. Sincethe epitaxial layer 60 finally constitutes the sheet member 14 (and alsothe frame 12), it has such a thickness that the flexible part 15 iselastically flexed and deformed when an acceleration is applied.Thereafter, the P-type impurity is introduced (for example, the impuritysuch as boron is diffused) into those portions which corresponds to thethird space, so that those portions 62 are formed which have the higherimpurity concentration.

FIG. 3(d): Then, the pieziresistors 64 and 66 which transduce theirresistance change due to the flexure into an electric signal are formedon portions of the epitaxial layer 60 which are formed into the flexibleparts which can flex when the acceleration is applied. They are formedby diffusing into such portions, the P-type impurity such as boron ofwhich conductivity type is opposite to that of the epitaxial layer 60.It is noted that the piezoresistor 66 may be used for offset or used asone of the piezoresitors which forms a bridge circuit.

FIG. 3(e): Then, the wiring parts 68 which are electrically connected tothe piezoresistors 64 and 66 are formed by the deposition and thethermal diffusion or the ion implantation and the annealing treatment.

FIG. 3(f): Then, the exposed surface of the epitaxial layer 60 and thesecond main surface of the substrate are covered with silicon nitridefilms 70. Thereafter, the silicone nitride film is removed from thatportion which corresponds to an opening 72 of the first space 36 inorder to form the first space. It is preferable that silicon oxide filmsare formed beforehand the formation of the silicon nitride films 70.

FIG. 3(g): Then, by using an alkaline solution such as a potassiumhydroxide solution through the opening 72 on the second main surface ofthe silicon substrate 50, the silicon substrate 50 is partially removedby the anisotropic etching so that the first space 36 which leads to thesacrificial layer 56, the side surface 30 of the support member 20 andthe side surface 34 of the weight 26 are formed. The anisotropic etchingis such that an etching speed is faster along the thickness direction ofthe silicon substrate 50 and slower along a direction perpendicular tothe thickness direction. Thus, since the sacrificial layer 56 extendsperpendicular to the thickness direction of the silicon substrate 50,the etching stops while the sacrificial layer is hardly etched.

FIG. 3(h): Then, a portion of the silicon nitride film 70 on the firstmain surface is removed, and the electrode 74 is formed by thedeposition or sputtering which is electrically connected to thepiezoresistor 64 or 66 through the wiring part 68.

FIG. 3(i): Then, the sacrificial layer 56 is removed by the isotropicetching in which the etching proceeds to all directions and an etchantis supplied through the first space 36, so that the sheet member 14 isformed from the epitaxial layer 60 in which member the both edges aresupported by the frame 12 of the epitaxial layer 60 and the weight 26 ishung from the center part 22 of the sheet member through the neck part28.

In the etching in this step, an acidic solution containing hydrofluoricacid may be used. When such isotropic etching is carried out, theetching speed is faster in the sacrificial layer 56 in which theimpurity concentration is high than in the epitaxial layer 60 in whichhe impurity concentration is low, and thus, only the sacrificial layer56 is selectively removed, whereby the second space is provided.Finally, the portions 62 which have been so formed in the step of FIG.3(c) that the impurity concentration thereof is high are removed by theisotropic etching subsequently to the removal of the sacrificial layer56, so that the third space 40 is formed which is defined by the sheetmember 14 and the frame 12. The third space 40 may be in the slit formcomposed of the spaces 39 and 43 as in the embodiment shown in FIG. 1.

It is noted that differently from the RIE (reactive ion etching), anedge portion includes a round corner when the isotropic etching is used,thus stress concentration is prevented at the edge when flexure isconverged near the edge, and thus there is an advantage in that asemiconductor acceleration sensor is provided of which life time isextended. It is of course possible to form the third space 40 using theanisotropic etching which engraves along one direction or the RIE whenthe round portion is not required.

In the above method according to the present invention, since theanisotropic etching is used for the formation of the first space, thedistance between the side surfaces of the support member and the weightcan be made as small as possible, namely the first space can be madethin, and also the first space can be located as outward as possiblewith respect to the substrate, so that the volume of the weight can belarger when a substrate having a fixed size is used (thus, the weightcan be heavier). Further, since the sacrificial layer is formed and thenremoved, the connection between the weight and the sheet member by thenarrow neck part, and thereby the distance can be longer from the centerof the flexible part to the frame when the substrate having a fixed sizeis used. Particularly, when the flexible part is substantially in theform of a beam as shown in FIG. 1, since in addition to the flexureconcentration in the flexible part, the length of the flexible part canbe longer, the sensitivity of the sensor is improved.

In the shown embodiment, that portion of the epitaxial layer except thecenter part 22 of the cruciform sheet member 14 and the weight upperpart 41 includes no substrate below it and is made of only the eptaxiallayer 60, and thus such a portion can substantially deform (or flex)when the acceleration is applied.

The operation of the semiconductor acceleration sensor shown in FIG. 1will be explained. When an acceleration 1 applied to the frame 12, theweight 26′ is displaced toward a direction which is opposite to thedirection of the acceleration application, so that the flexible part 15of the sheet member 14 flexes, whereby the piezoresistor 42 (or 64)formed in that part flexes and its resistance changes.

In this case, that portion of the sheet member 14 which is substantiallyelastically flexible is a both-end supported beam structure in whichboth ends are supported by the frame and the weight is connected to thecenter of the structure, and the weight is supported by the four beams(flexible parts 15). Therefore, the beams flex upon the accelerationapplication along any direction with respect to X, Y and Z axes whichintersect with one another at a right angle, and the accelerationincluding the three axis components can be sensed.

The other piezoresistor 66 having the same structure as that of thepiezoresistor 64 is formed on the top surface of the frame 12 asdescribed above, and piezoresistors 64 and 66 are connected to eachother so as to form a bridging circuit (not shown). By measuring aresistance of the piezoresistor 64 through the circuit, the accelerationis detected which is composed of the three axis components.

In the production method for the semiconductor acceleration sensor asdescribed above, the side surface 34 of the weight 26 is formed byanisotropically etching the silicon substrate 50 from its second mainsurface, the sheet member 14 is formed by isotropically etching andremoving the sacrificial layer 56 which is so formed that it partiallysurrounds the center part 22 of the silicon substrate 50, and suchisotropic etching does not proceed as to the epitaxial layer 60 which isformed into the sheet member 14 because of the low impurityconcentration of the eptaxial layer 60. Therefore, the thickness of thesheet member 14 is precisely controlled so that the acceleration sensorhaving the both-end supported beam structure is stably produced whichhas less sensitivity dispersion.

In addition, although in the shown embodiment, the actually elasticallydefomable portion is in the beam form, it may be wider and/or shorterwhen the sensitivity is not so important.

Further, the conductivity type of the substrate and the epitaxial layer60 is an N-type in the above embodiment, it may be a P-type, in whichcase the piezoresistor 64 may be of an N-type.

Next, an embodiment will be explained in which the removal of thesacrificial layer with etching is carried out by etching and removingthe sacrificial layer through the third space so as to form the secondspace. The formation of the third space may be carried out at anyappropriate time, and for example, it may be before or after, orsimultaneously with the formation of the first space.

First, the predetermined sacrificial layer 88 and the epitaxial layer 82are provided to the substrate 96 similarly to FIGS. 3(a) to (c). Then,as shown in FIG. 4(a), the epitaxial layer 82 is so masked that only theportion 84 of the epitaxial layer 82 excluding portions which are to befinally the sheet member 80 (including the center part 92 and theflexible parts 106) and the frame 90 is subjected to the etching, andthen the portion 84 is removed by the RIE or the anisotropic etching sothat the third space 86 is formed as an etchant introduction port andthe sacrificial layer 88 is exposed at the bottom of the space 86, asshown in FIG. 4(b). Thereafter, the sacrificial layer 88 is removed bywet isotropic etching so that the sheet member 80 and the frame 90 areformed and also the weight 94 including the neck part 93 and the supportmember 95 are formed as shown in FIG. 4(c). In this way, when theetchant introduction port is formed in the portion of the epitaxiallayer excluding the portions which is to be the sheet member and theframe, the sacrificial layer 88 as the high impurity concentration layeris directly etched, and also since the etching proceeds along thedirection shown with the arrow of FIG. 4(b) below the elongated portionof the sheet member (i.e. the flexible part 106), the etching path isshortened, which results in the shorter etching removal period. In theabsence of the etchant introduction port, the etching have to proceedalong a direction which is perpendicular to the arrow and from the outerperiphery of the weight 94 toward the neck part 93. It is noted that theembodiment shown in FIG. 4, the first space has been already formed.

In the embodiment shown in FIG. 4, the cross section of the third space86 along the eptaxial layer 82 is of a substantially square except thata corner portion 97 near the center part 92 is inwardly convex. Thecross section may be of any appropriate shape. Examples of the shape areschematically shown in the top views in FIGS. 5(a) to (l). Furtheradditional examples are schematically shown in the partially cut-awayperspective views in FIGS. 6(a) and (b).

For example, the etchant introduction port 86 (indicated with theslanted lines) is formed by etching and removing the epitaxial layer 82except those portions which are to be the sheet member 80 and the frame90, and then the etchant is supplied through the port 86 so as to etchand remove the sacrificial layer 86. Then, a residence phenomenon of theetchant is deleted so that the convection of the etchant proceeds fast,and thereby that effect is advantageously avoided which is based on thechange of etchant composition due to the self-catalytic decomposition ofnitric acid contained in the etchant in a locally confined space. Thus,the sheet member 80 is precisely formed without degrading theselectivity of the sacrificial layer 88 over the epitaxial layer 82.Further, as to the etching of the sacrificial layer 88 below the flexpart 106, the etching can proceed along the direction (i.e. thedirection of the arrow shown in FIG. 4(b) or FIG. 6(b)) which isperpendicular to the longitudinal direction 104 of the flexible part106, the etching period can be shortened compared with the etching alongthe longitudinal direction 104 of the flexible part 106. It is notedthat the etchant introduction port 86 is formed simultaneously with theformation of the first space 100 with the anisotropic etching, theintroduction port 86 is produced without an additional step.

FIGS. 5(b) to (l) are schematic plane views of the sensors for thesemiconductor acceleration sensor according to the present inventionwhen seeing them from the above thereof, and show the shapes and thearrangements of the etchant introduction ports.

In FIG. 5(b), the corners of the shape of the etchant introduction port86 shown in FIG. 5(a) when seeing it from the above are rounded, whichimproves the mechanical strength against the stress concentration of theflexible part 106 of the sheet member in the form of the beam at the endof the part 106 which reaches the frame 90.

In FIG. 5(c), the etchant introduction ports as shown in FIG. 5(a) areformed in only those portions which are adjacent to the flexible parts106. In this embodiment, the sacrificial layer 13 is etched and removedbelow and near the flexible parts 106, and the epitaxial portions 108(which is enclosed by the etchant introduction port 86 and the frame 90)is not removed so that the substrate is left below the portions when thesacrificial layer is absent, whereby the sensitivity is improved becauseof the volume increase of the weight. It is noted that it is required toform slits at least partially inside the flexible parts 106 and betweenthe portions 108 and the frame 90 using for example the RIE so that theflexible parts 106 have additional flexibility.

In FIG. 5(d), the etchant introduction ports 86 shown in FIG. 5(c) isdivided into a plurality of the rectangular portions, and the similareffect to that in the embodiment of FIG. 5(c) is achieved. The flexibleparts 106 are partially connected to the epitaxial layer portions 108which are connected to the frame 90. When a wafer is rotated at a highspeed upon coating with a resist, the flexure or breakage of the sheetmember 80 because of for example the high viscosity of the resist isprevented, so that this embodiment is better in the mechanical strengthas to the handling aspect (i.e. workability). As in the embodiment ofFIG. 5(c), the slits are required to be formed in the flexible parts 106so as to confer the additional flexibility. In the shown embodiment, aplurality of the rectangular etchant introduction ports 86 are formed.However, there is no specific limitation to this, and for example theetchant introduction port may be an oval shape.

FIGS. 5(e) to (h) correspond to the embodiments of FIGS. 5(a) to (d)respectively in which the etchant introduction ports 110 are furtherformed inside the flexible parts 106, and thereby the sacrificial layer88 is etched from the center portions and the sides of the flexibleparts 106 so that the etching period is shortened. The shape of theetchant introduction port when seeing it from the above may be any shapesuch as a circle, oval, rectangle, square, square having round cornersand so on, but the circle, the oval and the square having the roundcorners are preferable when the stress concentration around the etchantintroduction port is considered. A plurality of the etchant introductionports 110 may be present along a centerline of the flexible part 106which is parallel to the longitudinal direction of the flexible part. Itis noted that the above descriptions as to FIG. 5(a) to (d) are alsoapplicable to FIG. 5(e) to (h) respectively when they are appropriate.

FIGS. 5(i) to (l) correspond to the embodiments of FIGS. 5(a) to (d)respectively in which the etchant introduction ports 112 are furtherformed along the substantially an entire length of the flexible parts106, and thereby the sacrificial layer 88 is etched from the centerportion and the sides of the flexible part 106 along a directionperpendicular to the longitudinal length of the flexible part 106 sothat the etching period is shortened. The shape of the etchantintroduction port 112 when seeing it from the above may be any shapesuch as an oval, rectangle, square, rectangular having four roundcorners and so on, but the oval and the rectangular having the fourround corners are preferable when the stress concentration around theetchant introduction port is considered. It is noted that the abovedescriptions as to FIG. 5(a) to (d) are also applicable to FIG. 5(i) to(l) respectively when they are appropriate.

In the embodiments shown in FIG. 5(a), FIG. 5(b), FIG. 5(e), FIG. 5(f),FIG. 5(i) and FIG. 5(j), the epitaxial layer 82 is removed by etchingexcept the sheet member 80 and the frame 90 as in the embodiment shownin FIG. 6(b). However, in the embodiments shown in FIG. 5(c), FIG.(d),FIG. 5(g), FIG. 5(h), FIG. 5(k) and FIG. 5(l), the epitaxial layer 82may be etched in only those portions which are adjacent to the frame 90so that the slit 87 is formed which connected to the first space and thethird space, as shown in FIG. 6(a), so that the weight 94 is madeheavier and thereby the sensitivity is improved. The flexible parts 106of the embodiments shown in FIGS. 5(e) to (l) correspond to the flexibleparts 106 of FIG. 6(a) in which the etchant introduction ports 110 or112 are further formed.

In the embodiments shown in FIG. 5(c), FIG. 5(d), FIG. 5(g), FIG. 5(h),FIG. 5(k) and FIG. 5(l), if a size of the etchant introduction port 86(especially the size of the portion 84) is so designed under theconsideration of the anisotropic etching properties that the etchingautomatically stops when the sacrificial layer 86 is reached,excessively-etching of the weight 94 is prevented, which otherwise wouldreduce the sensitivity. Such designing can be carried out by controllingan opening size of the mask for the anistropic etching.

In the embodiments shown in FIGS. 5 and 6, although the weight iscarried by the four flexible parts 106 in the form of the beams (seeFIG. 6(a)) or the eight flexible parts 106 in the form of the beams (seeFIG. 6(b)), the number of the beams is not limited to such embodiments.Any number (for example, twelve beams and sixteen beams) of the beamsmay be employed to carry the weight.

In the embodiment shown in FIG. 6(a), the eptaxial layer 82 forms thesheet member 80 including the flexible parts 106, the center part 107and weight upper parts 91, while the embodiment shown in FIG. 6(b) hasno weight upper parts.

The formation of the third space by providing the second high impurityconcentration layer in the epitaxial layer will be explained withreference to FIG. 7.

FIG. 7 schematically shows in the cross sectional views, the steps forthe production of the transducer element for the semiconductoracceleration sensor according to the present invention, and FIG. 8schematically shows in the partially cut-away perspective views, somestages during the steps of FIGS. 7(b) to (i).

FIG. 7(a): On the single crystal silicon substrate 150 as an N-typesemiconductor substrate having a thickness of for example 400 to 600 μmand an orientation of crystal plane (crystal orientation) of (100) isprovided a silicon oxide film 152 by for example the thermal oxidation,and then the openings 154 are formed by etching the silicon oxide film152 through a mask of a photoresist (not shown) having a predeterminedpattern followed by the removal of the photoresist through for examplethe plasma ashing. The openings 154 are formed on those portions whichextend outward from four sides surrounding the generally rectangularcenter part 156 (thus, the portions are elongated ones which partlysurround the center part), and on which the flexible parts (concretely,the beam parts) are to be formed, and also on vicinities alonglongitudinal directions of those portions. Thus, the width of theflexible part 158 is smaller than that of the opening 154.

The opening 154 is not limited to the shown embodiment, and it may beformed in that portion which entirely surrounds the center part 156 ofthe single crystal silicon substrate 150 (i.e. that portion is annular).

Then, using the silicon oxide film 152 having the openings 154 as themask, the sacrificial layers 160 are formed as high impurityconcentration layers in which the P-type impurity such as boron (B) ispresent at a high concentration by the deposition and the thermaldiffusion or the ion implantation and the annealing treatment of theP-type impurity. The concentration of the P-type impurity in thesacrificial layer 160 is desirably for example not less than 1.0×10¹⁷cm⁻³ and below its solid solubility (limit).

FIG. 7(b): Then, the silicon oxide film 152 is removed by etching.Thereafter, on the side where the P-type sacrificial layer 160 of thesingle crystal silicon substrate 150 is formed is formed the eptaxiallayer 162 which has such a thickness that the flexible part 158 to beformed can elastically flex when the acceleration is applied, and thenthe piezoresistors 164 are formed in the predetermined portions of thecorresponding flexible parts 158 of the epitaxial layer 162 using aresist having the predetermined pattern as a mask and also using thedeposition and the thermal diffusion or the ion implantation and theannealing treatment of the P-type impurity such as boron (B) (see FIG.8(a)).

FIG. 7(c): Similarly, the diffusion wirings 166 are formed which areelectrically connected to the piezoresistors 164 by the deposition andthe thermal diffusion or the ion implantation and the annealingtreatment of the P-type impurity, and then the photoresist is removed.

FIG. 7(d): Then, the P-type impurity layers 168 which reach the buriedP-type impurity layer 162 are formed adjacently to those portions whichare to be the sheet member of the eptaxial layer 162, especially theflexible parts 158 by the deposition and the thermal diffusion or theion implantation and the annealing treatment of the P-type impurity, andthen the photoresist is removed (see FIG. 8(b)).

In this embodiment, the impurity layers 168 are formed adjacently to theflexible parts 158, to which the embodiment is not limited. The impuritylayers of the epitaxial layer may be so formed that they are connectedto the sacrificial layer 160 excluding the flexible parts 158, thecenter part 188 and frame 186. Further, in this embodiment, although theimpurity layers 168 are formed after the piezoresistors 164 and thediffusion wirings 166 have been formed, the piezoresistors 164 and thediffusion wirings 166 may be formed after the impurity layers 168 havebeen formed.

FIG. 7(e): Then, on the single crystal silicon substrate 150 and theepitaxial layer 162 are formed the silicon oxide films 170, on whichprotection films 172 such as silicon nitride films are formed. These twokinds of films are advantageous in that their camber directions areopposite to each other, which serves to keep the flatness of thesubstrate. Thereafter, the openings 176 are formed in predeterminedportions which correspond to the outer periphery of the weight 174described below by etching the silicon oxide film 170 and the protectionfilm 172 thereon using the photoresist (not shown) as a mask having thepredetermined pattern, and then the resist is removed.

FIG. 7(f): Then, the silicon substrate 150 is anisotropically etchedwith an alkaline etchant solution (such as a KOH solution) using as amask the protection film 172 having the openings 176, so that the firstspace 178 reaching the buried P-type sacrificial layer 160 is formed.

FIG. 7(g): Then, portions of the silicon oxide film 170 and theprotective film 172 on the P-type impurity layers 168 are removed byetching so as to provide openings (not shown) which are used for theformation of the third space, and through which an etchant based onhydrofluoric acid/nitric acid is supplied to form the etchantintroduction ports 180 as the third space.

FIG. 7(h): Then, through the etchant introduction ports 180 is suppliedan etchant based on hydrofluoric acid/nitric acid so that the buriedP-type sacrificial layers 160 are removed to form the second space 182(see FIG. 8(c)). As the etchant based on hydrofluoric acid/nitric acidherein, an etchant containing hydrofluoric acid: nitric acid:aceticacid=1:1 to 3:8 (50% hydrofluoric acid aqueous solution:69% nitric acidaqueous solution acetic acid, based on volume) may be used.

It is noted that the etching of the sacrificial layer 160 may be carriedout through the first space 178 and the third space 180.

FIG. 7(i): Then, the predetermined portions of the silicon oxide film170 and the protection film 172 thereon which portions are on thediffusion wirings 166 are removed by etching so as to provide contactholes which are filled, and then metal (such as aluminum) wirings 184are so formed that they are electrically connected to the piezoresistors164 through the diffusion wirings 166, and thereafter, the silicon oxidefilm 170 on the silicon substrate 150 and the protection film 172thereon are removed by etching (see FIG. 8(d)).

Finally, those portions of the epitaxial layer 162 which excludeportions to be the flexible parts 158, the center part 188 and the frame186, and optionally a portion of the single crystal silicon substrate150 below them are removed by the RIE (reactive ion etching), wherebythe sheet member (158+188) of which all ends are supported by the frame,and the weight 174 is hung from the center part 188 is provided,resulting in the element according to the present invention (see FIG.8(e)). In the element, the interfaces between the flexible parts 158 andthe frame 186 as well as the interfaces between the flexible parts 158and the center part 188 are preferably processed so as to have the edgeswith the rounded corners (or sides) so as to avoid the stressconcentration.

In this embodiment, the etchant introduction ports 180 are formed inthose portions which are adjacent to the flexible parts 158 of theepitaxial layer in the form of the beams, and the etchant is suppliedsuch ports to remove the buried P-type sacrificial layer 160 by etching,so that the effect is avoided which is based on the change of theetchant composition due to the self-catalytic decomposition of nitricacid contained in the etchant in the locally confined space. Thus, theflexible parts 158 are precisely formed without degrading theselectivity of the buried P-type sacrificial layer 160 over theepitaxial layer 162.

Also, since the P-type impurity layer 168 is of the high impurityconcentration as in the P-type impurity layer 160, the etching removalsof the impurity layers 168 and 160 can be carried out successively, sothat the step can be shortened.

Further, in this embodiment, since the etching can be carried out notalong the longitudinal direction of the flexible part 158, but along adirection which is perpendicular to the longitudinal direction of theflexible part 158, the etching path can be shortened.

Next, an embodiment will be explained wherein the first space iscomposed of two parts.

FIG. 9 shows a schematic cross sectional view of one embodiment of theelement 200 for the semiconductor acceleration sensor according to thepresent invention, and is substantially similar to for example the crosssectional view of FIG. 3(i) except that the shape (or form) of the firstspace 202 is different.

As easily seen from FIG. 9, the element 200 includes the first space 202composed of the first part 204 which is formed by a mechanical orchemical process and the second part 206 which is formed by theanisotropic etching. Clearly from FIG. 9, the side surface 210 of theweight 208 and the side surface 214 of the support member 212 form sucha taper that the angle (θ₁) of the first part is smaller than the angle(θ₂) of the second part, each angle being defined by the tow sidesurfaces.

Upon the formation of the first space 202, when the anisotropic etchingis used from the beginning of the formation, the angle formed by theside surface of the weight and the support member is θ₂, and the etchinghas to proceed up to such a depth that the sacrificial layer is reached.Thus, the opening 216 should be larger as shown with broken lines, sothat there occurs a problem that the volume of the weight is decreased.Such a problem is overcome in this embodiment.

That is, in the element as shown, the volume of the weight can beincreased without enlarging an area of the opening 216 for the firstspace. This means that the sensitivity of the acceleration sensor can beimproved without enlarging the chip area of the sensor. It is noted thatalthough in the shown embodiment, the first space is composed of the twoparts, the first part may be ground mechanically or chemically so as tofurther divide it into a plurality of sub-parts provided that thetapering angle of the sub-part is smaller than the tapering angle of thesecond part (θ₂)

Concretely, upon the formation of the first space as the above, thesemiconductor substrate 218 is mechanically ground up to around itsmidpoint so as to form the first part 204. The shape of the opening ofthe first part 204, especially the distance between the both sidesurfaces should be such that a sufficient second part opening is ensuredfor the formation of the second part 206 in the next step.

As to the mechanically grinding for the formation of the first part, forexample, using a dicing saw or collision of particles at a high speed(such a sandblast manner) may be used. The sandblast manner blows finesand particles against an object at a high pressure and thereby amaterial is removed from the object. Alternatively, the first part maybe formed using a chemical reaction, and for example, the reactive ionetching (RIE) may be used.

Then, the second part 206 is formed by the anisotropic etching using analkaline solution such as a potassium hydroxide aqueous solution. Theetching is carried out so that when the sacrificial layer is present inthe second space 220, it proceeds up to the sacrificial layer whichserves as an etching stop layer. When the sacrificial layer is notpresent, the etching is stopped when it reaches the second space 220.The first part 204 functions as a mask and an etchant introduction portfor the formation of the second part 206. As the etchant, in addition tothe potassium hydroxide aqueous solution, ethylenediamine pyrocatechol,hydrazine and so on may be used.

The mechanical grinding is advantageous in that the removal speed islarger compared with the etching so that a thicker substrate can beworked, which makes the volume (thus heft) of the weight larger. Thereactive ion etching is one of the semiconductor processing techniques,and advantageous in that it is used in the same environment as used inother processing for the production of the element, and also that θ₀ canbe smaller relatively to the mechanical grinding (namely, grinding canbe carried out at an angle which is closer to a right angle with respectto the substrate 218), so that the opening 216 is made smaller and θ₁can be substantially 0°.

The embodiment wherein the first space is made of the two parts will beexplained in detail with reference to FIG. 10.

FIG. 10(a): As the semiconductor substrate 218, an N-type substratehaving an orientation of crystal plane of (100) is used. Thesemiconductor substrate 218 desirably has an impurity concentration ofnot more than 1.0×10¹⁷ cm⁻³. The thickness of the substrate is thicker alittle (for example, a thickness of about 1000 μm) relatively to theconventionally used one.

First, in a diffusion step for the formation of the sacrificial layer230, the deposition and the thermal diffusion or the ion implantationand the annealing treatment are carried out. The impurity such as boronis used herein and doped to a high concentration. The diffusion depth iscontrolled depending on the application. The sacrificial layer 230 maybe an N-type high impurity concentration layer while using antimony,phosphorous and so on. The sacrificial layer 230 functions as an etchingstop and the sacrificial layer itself in the present invention.

FIG. 10(b): The eptaxial layer 232 is formed on the substrate 218through the epitaxial growth. The epitaxial layer 232 constitutes theframe and the sheet member of the flexure transducer element. Since thelayer 232 is formed by the epitaxial growth, its thickness is controlledeasily and precisely.

FIG. 10(c): Then, the piesoresistors 234 are formed in those portions ofthe epitaxial layer which are to become the flexible parts by thethermal diffusion or the ion implantation and the annealing treatmentusing the P-type impurity such as boron.

FIG. 10(d): Then, the diffusion wirings 236 which output the change ofresistance of the piezoresistors 234 are formed in those portions of theepitaxial layer 232 which are to become the flexible parts by thedeposition and the thermal diffusion or the ion implantation and theannealing treatment using the P-type impurity such as boron. Theimpurity concentration is larger than that in the piezoresistorformation step (FIG. 10(c)).

FIG. 10(e): Then, the protection mask 238 which protects the epitaxiallayer 232, the piezoresistors 234 and the diffusion wirings 236 as wellas the formation mask 240 which is used to form the weight 250 areformed. Both masks are preferably made of a silicon nitride film and/ora silicon oxide film. Subsequently, the first part 242 is formed bymechanically grinding the semiconductor substrate 218 for example up toaround the midpoint thereof. As the mechanical grinding means, a dicingsaw is herein used. The size of the opening 244 of the first part 242 issuch that the necessary opening 245 is ensured for the next step for theproduction of the second part 246.

FIG. 10(f): Then, the second part 246 is formed by anistropicallyetching using the alkaline aqueous solution such as a potassiumhydroxide. T he etching is carried out up to the sacrificial layer 230which functions as the etching stop layer. The first part 242 functionsas an etching mask and an etching solution introduction port for theformation of the second part 246.

FIG. 10(g): Then, the predetermined portions of the silicon oxide filmand the silicon nitride film on the diffusion wirings are removed so asto form contact holes, and the metal wirings 248 are so formed that theyare in contact with the diffusion wirings 236 by sputtering ordeposition. When aluminum is used, a thermal treatment such as sinteringis desirably carried out. For the metal wirings 248, gold, chromium andso on may be used.

FIG. 10(h): Finally, the sacrificial layer 230 which has functioned asthe etching stop layer is removed by etching so as to form the secondspace 254. For the etching herein, for example a solution containinghydrofluoric acid:nitric acid:acetic acid=1:1 to 3:8 is used. In thiscase, since the etching speed in the low impurity concentration(diffusion) layer having an impurity concentration not larger than1.0×10¹⁷ cm⁻³ is reduced to about 1/150 of the etching speed of thediffusion layer having an impurity concentration above 1.0×10¹⁷ cm⁻³,only the low impurity concentration (diffusion) layer can be selectivelyleft. That is, the sacrificial layer 230 in which the impurity isdiffused at a high concentration can be selectively etched so that theweight 250 and the support member 252 is separated.

When as described above, the first space is divided into the pluralparts, for example two parts and the anisotropic etching is applied forthe final part while the mechanical grinding or the RIE is applied forthe other parts so that those parts are formed, the volume of the weight250 can be made large without enlarging the area of the opening 244,whereby the sensitivity of the acceleration sensor can be improvedwithout enlarging a chip area.

Then, an embodiment in which the wiring protection film is formed in theproduction of the acceleration sensor will be explained with referenceto FIG. 11 which schematically shows the production steps of theacceleration sensor.

FIG. 11(a): On the single crystal silicon substrate 300 as an N-typesemiconductor substrate having a thickness of for example 400 to 600 μmand an orientation of crystal plane of (100) is provided a silicon oxidefilm 302 by for example the thermal oxidation, and then the openings 304are formed by etching the oxide film 302 with a mask of a photoresist(not shown) having a predetermined pattern followed by the removal ofthe photoresist through for example the plasma ashing. The openings 304are formed on those portions which surround the generally rectangularcenter part 306 of the single crystal silicon substrate 300.

Then, using the silicon oxide film 302 having the openings 304 as themask, the P-type sacrificial layers 308 are formed in which the P-typeimpurity such as boron (B) is doped by the deposition and the thermaldiffusion or the ion implantation and the annealing treatment of theP-type impurity. It is noted that in place of the silicon oxide film,for example a silicon nitride film is formed, and then the depositionand the thermal diffusion or the ion implantation and the annealingtreatment may be carried out using the nitride film as a mask.

FIG. 11(b): Then, the silicon oxide film 302 is removed by etching.Thereafter, on the side where the buried P-type impurity sacrificiallayer 308 is formed in the single crystal silicon substrate 300 isformed the N-type eptaxial layer 310, and then the piezoresistors 312are formed in the predetermined portions of the flexible parts 338 whichare to be formed from the epitaxial layer 310 using the deposition andthe thermal diffusion or the ion implantation and the annealingtreatment of the P-type impurity. It is noted that since the epitaxiallayer 310 is finally formed into the sheet member including the flexibleparts 338, it is formed to have a thickness which allows the elasticflexure upon the application of the acceleration.

FIG. 11(c): Then, the diffusion wirings 314 are so formed that they areelectrically connected to the piezoresistors 312 by the deposition andthe thermal diffusion or the ion implantation and the annealingtreatment for the high P-type impurity concentration, and then thesilicon oxide films 316 are formed on the single crystal siliconsubstrate 300 and the exposed epitaxial layer 310.

FIG. 11(d): Then, the protection films 318 such as silicon nitride filmsare formed on the silicon oxide films 316 by for example the CVDprocess, and then a portion of the protection film 318 and the siliconoxide film 316 is removed by etching, for example, the RIE, so that theopening 320 for the first space 322 is formed which surrounds the weight336 which will be explained below.

FIG. 11(e): Then, the single crystal silicon substrate 300 isanisotropically etched using as a mask the protection film 318 havingthe opening 320 and also using an alkaline based etchant such as apotassium hydroxide aqueous solution, so that the first space 322 isformed which reaches the P-type sacrificial layer 308.

FIG. 11(f): Then, the pre determined portions of the silicone oxide film316 and the protection film 318 which are on the diffusion wirings 314are removed by etching, and then the metal wirings 324 (made of forexample aluminum) and the electrode pads (not shown) are so formed thatthey are electrically connected to the diffusion wirings 314, and thenthe wiring protection film 326 such as a chromium film, a siliconnitride film or a fluoroplastic film is formed on the side of the singlecrystal silicon substrate 300 having the metal wirings 324.

It is noted that when the conventional aluminum is used for the metalwirings 324, an alloy spike problem may occur above 500° C. Therefore,it is desirable that the wiring protection film 326 of the siliconnitride film is applied in low temperature growth using for example theplasma CVD method.

FIG. 11(g): Then, portions of the wiring protection film 326, theprotection film 318, the silicon oxide film 316 and the epitaxial layer310 are removed by the RIE, the anisotropic etching or the isotropicetching, so that the third spaces 328 including the etchant introductionport which reach the buried P-type sacrificial layer 308 are formed.Then, the etchant of an acidic solution containing hydrofluoric acid(hydrofluoric acid:nitric acid:acetic acid=1:1 to 3:8) is suppliedthrough the etchant introduction ports so as to isotropically etch andremove the buried P-type sacrificial layer 308 and the second space 330is formed, whereby the sheet member 338 is formed of which ends areconnected to the frame 334 supported by the support member 332 made ofthe substrate 300 and made of the epitaxial layer 310, and to whichcenter part the weight 336 is connected.

FIG. 11(h): Then, the wiring protection film 326 as well as thoseportions of the silicon oxide film 316 and the protection film 318 whichare on the side of the bottom surface of the weight 338 are removed byetching. Finally, the stopper (or the bottom cover) 342 which containsthe recess part 340 in a portion thereof corresponding to the weight 336is connected to the support member 332 by for example anode bonding,which results in the acceleration sensor according to the presentinvention.

Another embodiment in which the wiring protection film is formed isshown in FIG. 12. Up to the formation of the silicon oxide film 316,this embodiment is similar to FIG. 11, and thus the explanations up tothe formation of the silicon oxide film 316 are omitted.

FIG. 12(a): Thereafter, the predetermined portions of the silicone oxidefilm 316 which are on the diffusion wirings 314 are removed by etching,and then the metal wirings 324 (made of for example aluminum) and theelectrode pads (not shown) are so formed by for example the sputteringor the vapor deposition that they are electrically connected to thediffusion wirings 314.

FIG. 12(b): Then, the wiring protection films 326 of the silicon nitridefilms are formed on the both silicon oxide films 316 by for example theCVD, and then a portion of the wiring protection film 326 and thesilicon oxide film 316 is removed by etching, for example, the RIE, sothat the opening 320 for the first space 322 is formed. It is noted thatthe wiring protection film 326 are so formed that it covers the metalwirings 324 and the electrode pads (not shown).

FIG. 12(c): Then, the single crystal silicon substrate 300 isanisotropically etched using as a mask the wiring protection film 326having the opening 320, so that the first space 322 is formed whichreaches the sacrificial layer 308.

FIG. 12(d): Then, portions of the wiring protection film 326, thesilicon oxide film 316 and the epitaxial layer 310 are removed byetching, for example the RIE, the isotropic etching or the anisotropicetching so that the third space 328 which reaches the sacrificial layer308 is formed. Through the third space 328, the etchant comprising anacidic solution which contains hydrofluoric acid and so on (a solutioncontaining hydrofluoric acid:nitric acid:acetic acid=1:1 to 3:8) issupplied so as to remove the sacrificial layer 308.

FIG. 12(e): Finally, the wiring protection films 326 are removed, andthen the stopper (or the bottom cover) 342 which contains the recesspart 340 in a portion thereof corresponding to the weight 336 isconnected to the support member 332 by for example anodic bonding, whichresults in the acceleration sensor according to the present invention.

It is noted that in the embodiment shown in FIG. 12, the wiringprotection films 326 are removed from the whole of the surfaces, but thepresent invention is not limited to this embodiment. Only those portionsof the wiring protection film 326 which are on the electrode pads may bethinned beforehand by pattern etching, and then the wiring protectionfilm 326 is etched over its entire surface to reduce the thickness ofthe film 326 after the etching of the sacrificial layer 308, so thatonly the electrode pads are exposed. In this way, the other portions butthe electrode pads are covered by the silicon nitride film, whichimproves the moisture resistance of the sensor element. The reasons whythe wiring protection film 326 on the electrode pads are thinnedbeforehand by the pattern etching are that the substrate hasirregularities on its surface after the etching of the sacrificial layer308 and also has a less strength so that a pattern processing (forexample, a photolithography step) becomes difficult, and that when theonly portions of the wiring protection film 326 on the electrode padsare thinned beforehand by the pattern etching, only the electrode padsare exposed by etching the wiring protection film 326 over its entiresurface without the pattern processing after the etching removal of thesacrificial layer 308.

In a further embodiment, the first space 322 is partially formed so thata portion 350 of the substrate is left between the sacrificial layer 308and the first space 322, then the sacrificial layer is removed, and thenthe left portion 350 of the substrate is removed. In this embodiment,even after the etching of the sacrificial layer 308, the weight 336 andthe support member 332 are not separated so that no breakage of thesubstrate occurs in this step, which greatly improves the yield of thesubstrate.

More concretely, such an embodiment will be explained with reference toFIG. 13.

FIG. 13(a): The first space 322 is formed by the anisotropic etchingusing the etchant of an alkaline solution such as a potassium hydroxide(KOH) solution. Upon this, the etching is stopped before the first space322 reaches the sacrificial layer 308 so that the portion 350 of thesingle crystal silicon substrate having a thickness of for exampleseveral tens of micrometers is left below the sacrificial layer 308.

FIG. 13(b): Then, the predetermined portions of the silicon oxide film316 and the protection film 318 on the diffusion wirings 314 are removedby etching, then the metal wirings 324 (made of for example aluminum)and the electrode pads (not shown) are so formed that they areelectrically connected to the diffusion wirings 314, and then wiringprotection film 326 such as a chromium film, a silicon nitride film or afluoroplastic film is formed on the side of the single crystal siliconsubstrate 300 which side contains the metal wirings 324.

FIG. 13(c): Then, as in the embodiment shown in FIG. 12, the third space328 is formed, through which the etchant of an acidic solutioncontaining hydrofluoric acid is then supplied so as to isotropicallyetch and remove the sacrificial layer 308 and thereby the second space330 is formed. Then, the portion 350 of the single crystal siliconsubstrate below the sacrificial layer 308 is removed by etching, forexample, the RIE or the anisotropic etching so that the first space 322is connected to the second space 330.

It is noted that the shape of the portion 350 of the single crystalsilicone substrate which is left below the buried sacrificial layer 308after the etching depends on the etching way, and the anisotropicetching using an alkaline etchant provides the larger tapering angle(θ₁) of FIG. 9 compared with the RIE. Thus, when an area which theelement occupies is fixed, the RIE provides the larger size for theweight. This also means that when the size of the weight is fixed, theRIE provides the smaller chip compared with the wet anisotropic etching.

In a further embodiment, after the silicone oxide film 316 and theprotection film 318 on a bottom surface of a portion which is to be theweight are removed by etching, the single crystal silicon substrateportion 350 left below the buried sacrificial layer 308 and the bottomsurface of the portion which is to be the weight bottom are removed bythe anisotropic etching using the alkaline etchant or the RIE whileusing the protection film 318 on the support member 332 as a mask. Inthis embodiment, the thickness of the weight 336 is reduced, and thestopper 342 having a flat form is connected to the support member 332 byfor example anodic bonding (see FIG. 13(d)). In this way, no recess parthas to be formed in the stopper 342, which delete the processing cost ofthe stopper, whereby resulting the reduced production cost of the chip.

Then, an embodiment will be explained in which the concentration of theimpurity in the surface of the sacrificial layer is smaller than that ofan inner side thereof.

FIG. 14 shows a formation method for the buried sacrificial layer havingan aimed final depth of 10 μm.

FIG. 14(a): First, a field oxide film 362 having a thickness of about12000 Å is formed on the surface of the N-type silicon substrate 360 byfor example the thermal oxidation. The oxide film is patterned aspredetermined using the photolithography and the etching so as to formthe openings 364.

FIG. 14(b): Subsequently, using the field oxide film 362 as a mask,boron as the P-type impurity is deposited and then thermally diffused ina nitrogen atmosphere at the surface of the silicon substrate 360 so asto form the P-type impurity high concentration layer 366 having a depthof about 5 μm. Then, the silicon oxide film 368 having a thickness ofabout 3500 Å is formed in the substrate surface in the opening 364 bythe wet oxidation or pyrogenic oxidation. For example, while theimpurity concentration at the surface of the high P-type impurityconcentration layer 366 in the case of only the thermal diffusion underthe nitrogen atmosphere is about 1×10²⁰ cm⁻³, the concentration isreduced to about 4×10¹⁹ cm⁻³ when the additional wet oxidation orpyrogenic oxidation is carried out.

FIG. 14(c): Then, the field oxide film 362 and the silicon oxide film368 are fully removed by the wet etching over the entire surfacefollowed by the deposition of the N-type epitaxial layer 370. At thisstage, boron diffuses into the epitaxical layer 370 through an interfacewith the silicon substrate 360 and a final buried diffusion layer 372 isformed.

In this embodiment, when a silicon substrate having an impurityconcentration of for example 1×10¹⁵ cm⁻³, boron diffuses into theeptaxial layer 370 by about 4-5 μm without the wet oxidation orpyrogenic oxidation while about 3.5 μm with the wet oxidation orpyrogenic oxidation, and also the thickness of inversion layer formed bythe auto-doping is about 5 μm without the wet oxidation or pyrogenicoxidation while being reduced to about 2.5 μm with the wet oxidation orpyrogenic oxidation. Further, the peak concentration in the inversionlayer is in the order of 10¹⁶ cm⁻³ without the wet oxidation orpyrogenic oxidation while being reduced to about the order of 10¹⁵ cm⁻³with the wet oxidation or pyrogenic oxidation.

FIG. 15 shows another embodiment in which the concentration of theimpurity in the surface of the sacrificial layer is smaller than that ofthe inner side thereof.

FIG. 15(a): First, a field oxide film 362 having a thickness of about5000 Å is formed on the surface of the N-type silicon substrate 360 byfor example the thermal oxidation. The oxide film is patterned aspredetermined using the photolithography and the etching so as to formthe openings 364.

FIG. 15(b): Subsequently, using the field oxide film 362 as a mask,boron as the P-type impurity is ion implanted in the surface of thesilicon substrate 360.

FIG. 15(c): By annealing under an oxygen atmosphere, the silicone oxidefilm 365 and the high P-type impurity concentration layer 366 below thefilm are formed.

It is known that a peak of an impurity profile along the thicknessdirection just after the ion implantation appears in a portion which isinner side a little from the implantation surface due to so-called thechanneling effect. The peak position from the surface is determined bythe kind of impurity and an acceleration energy upon the implantation.For example, when boron is ion-implanted with an acceleration energy of100 keV, the peak appears at a point which is about 0.25 μm inside fromthe implantation surface Considering a fixed peak concentration, animpurity concentration at the surface is lower when the peak position isdeeper. In the embodiment shown in FIG. 15, anything such as aprotection oxide film is not formed on the surface of the opening 364 ofthe substrate, and the silicon substrate 360 is exposed. Since the ionimplantation is carried out into such a surface, when compared with thepresence of for example the protection film in the surface of thesilicon substrate, the peak of the impurity profile is located at adeeper position from the surface and simultaneously the surface of thesubstrate has a lower impurity concentration. Even when the diffusiontoward the inside of the substrate proceeds by the annealing treatmentthereafter, since the concentration profile is not changed, only thesurface P-type impurity concentration can be suppressed lower whilekeeping an overall impurity concentration in the high P-type impurityconcentration layer 366 around 1.0×10²⁰ cm⁻³ by properly selecting theion-implantation conditions and the annealing treatment conditionsthereafter.

It is noted that when the annealing treatment is carried out under anitrogen atmosphere, the sacrificial layer remains exposed in theopening, but when the annealing treatment is carried out under an oxygenatmosphere, the opening is covered with the silicon oxide film.Implanted impurity is likely to escape into the oxide film, so that theimpurity concentration in the substrate surface is preferably lowercompared when the oxide film is absent.

FIG. 15(d): Then, the field oxide film 362 and the silicon oxide film365 are fully removed from the entire surface of the substrate, and thenan N-type epitaxial layer 370 is deposited thereon. At this stage, borondiffuses into the eptaxial layer 370 through the interface with siliconsubstrate 360 so that the final sacrificial layer 372 is formed.

FIG. 16 shows a further formation method for the sacrificial layerhaving an aimed final depth of 10 μm.

FIG. 16(a): First, a field oxide film 362 having a thickness of about12000 Å is formed on the surface of the N-type silicon substrate 360 byfor example the thermal oxidation. The oxide film is patterned aspredetermined using the photolithography and the etching so as to formthe openings 364.

FIG. 16(b): Subsequently, using the field oxide film 362 as a mask,boron as the P-type impurity is deposited on the surface of the siliconsubstrate 360 and then thermally diffused in an oxygen atmosphere, andthe silicon oxide film 365 in the opening 364 and the high impurityconcentration layer 363 below it a reformed.

FIG. 16(c): Similarly, using the field oxide film 362 as a mask,phosphorous as the N-type impurity is implanted.

FIG. 16(d): By annealing treatment in a nitrogen atmosphere, the highP-type impurity concentration layer 366 having a depth of about 5 μm isformed. In this step, the ion implantation conditions for phosphoroushave to be so optimally selected that no inversion of the conductivitytype occurs in the high P-type impurity concentration layer 366.

FIG. 16(e): Then, the field oxide film 362 and the silicon oxide film368 are fully removed by the wet etching over the entire surfacefollowed by the deposition of an N-type epitaxial layer 370. At thisstage, the impurity diffuses into the epitaxical layer 370 through theinterface with the silicon substrate 360 and the final buried diffusionlayer 372 is formed. Since boron as the P-type impurity and phosphorousas the N-type impurity are both present near the surface of thesacrificial layer 372, each impurity escapes into the atmosphere uponthe formation of the epitaxial layer and trapped into the eptaxiallayer, both are compensated with each other, which suppress theformation of the inversion layer. Also, each impurity diffuses into theepitaxial layer side 370 through the surface of the silicon substrate360, the both are compensated with each other, so that the depth of theP-type impurity layer formed in the eptaxial layer is suppressed.

In a further embodiment, when the sacrificial layer is formed in thesubstrate, the impurity concentration of at least the epitaxial layerselected from the eptaxial layer and the substrate is made larger thanthe concentration of the impurity of the sacrificial layer which can betaken into the eptaxial layer through auto-doping upon the epitaxialgrowth.

Concretely, in the case in which the impurity concentration of the peakin the inversion layer as the sacrificial layer to be formed is intendedto be suppressed to about 7×10¹⁵ cm⁻³, for example when the epitaxiallayer was grown using the silicone substrate having an impurityconcentration of 1×10¹⁵ cm⁻³, and it was observed that the concentrationof the impurity which is actually taken into the epitaxial layer isabout 8×10¹⁵ cm⁻³ due to the auto-doping, another silicon substratehaving an impurity concentration (for example 1×10¹⁶ cm⁻³) which islarger than that of the above used silicon substrate is used for theepitaxial growth which provides the impurity concentration of 1×10¹⁶cm⁻³ of the epitaxial layer. It is noted that when the impurityconcentration of about 1×10¹⁵ cm⁻³ is required at an outermost surface(the surface on which the piezoresistor is formed) of the eptaxiallayer, the eptaxial growth may be so carried out using a siliconsubstrate having an impurity concentration of 1×10¹⁶ cm⁻³ that theimpurity concentration is continuously changed from 1×10¹⁶ cm⁻³ to1×10¹⁵ cm⁻³ when the epitaxial layer is formed.

Then, the formation of the porous silicon layer as the sacrificial layerwill be explained concretely with reference to FIG. 17.

After the silicon oxide film 402 is formed on one of the main surfacesof the semiconductor substrate 400 (for example, a single crystalsilicon substrate), an opening 404 is formed on a portion where thesacrificial layer is to be formed. Through the opening 404 is diffusedeither the P-type (for example boron) or the N-type (for examplephosphorous) impurity so as to form the buried layer 406. As shown inFIG. 17, such a substrate is placed as a diaphragm (or a partitionmembrane) in the electrolysis vessel 410 which contains an electrolytesolution 408 comprising for example hydrofluoric acid, and the poroussilicon layer 406′ as the sacrificial layer is formed by anodicoxidation.

In this step, the substrate is preferably enclosed by a protection filmsuch as a silicon oxide film except the buried layer 406. The siliconsubstrate 400 is placed between the two platinum electrodes 412 and 414to which a direct current source is applied from the outside. By theapplication of the power from the outside, a fluoride ion is generatedin the electrolyte, which reacts with a silicon atom of the buried layer406 as the sacrificial layer to produce silicon tetrafluoride (SiF₄) andhydrogen. Thereby, a portion of the impurity layer is dissolved, so thatfine pores are formed in the impurity layer, resulting in the poroussilicon layer. Thereafter, the substrate is washed with water and thendried for the next treatment.

It is noted that in place of the silicon oxide film 402, a siliconnitride film or a fluoroplastic material may be used for the mask of theelectrolyte solution treatment. The porous silicon thus produced can beused for the element according to the present invention.

The present invention provides the element for in addition to thepiezoresistor-type acceleration sensor, the electrostaticcapacitance-type acceleration sensor, and also the production method forthe same as well as the sensor in which the element is used. The elementused for the electrostatic capacitance-type acceleration sensor issubstantially different only in that the electrode for the electrostaticcapacitance measurement is used in place of the piezoresistors.Therefore, the constitutions of the element for the electrostaticcapacitance-type acceleration sensor and also the production method forthe same are obvious for those skilled in the art according to the abovedescriptions as to the element for the piezoresistor-type accelerationsensor of the present invention. Also, it is obvious for those skilledin the art that the electrostatic capacitance-type acceleration sensoris provided by connecting the top cover onto the element which coverincludes the electrode opposed to the capacitance measurement electrodeof the element. The concrete arrangement of the electrodes onto theelement may be for example the same as in the embodiment shown in FIG.21, or as shown in FIG. 1 in which the electrodes 734 are arranged (onlyone electrode is shown with the broken line).

What is claimed is:
 1. A flexure transducer element which is used in anacceleration sensor for sensing an acceleration applied theretocomprises (1) a frame having an upper surface and a lower surface, (2) asheet member which has a plurality of flexible parts and a center part,each flexible part extending between at least a portion of an inner edgeof the frame and the center part and being integrally connected to them,(3) a weight which has a neck part integrally connected to the centerpart of the sheet member and which is hung from the sheet member throughthe neck part, and (4) a support member which supports the lower surfaceof the frame and of which inward side surface of said support memberfaces to a side surface of the weight through a first spacetherebetween, the sheet member further includes a weight upper partwhich is located on the upper surface of the weight, and the upperweight part is integral with the weight, a second space which iscontinuous with the first space is defined between each flexible part ofthe sheet member and the weight, a third space is defined between theframe and the sheet member and/or through the sheet member, the frameand the sheet member are connected to each other and the sheet memberand the weight are connected to each other in such a manner that, whenthe acceleration is applied to the element, at least two flexible partsare elastically deformed so that the weight is displaced relatively tothe frame, the weight and the support member are formed of asemiconductor substrate, the second space is formed by removing asacrificial layer which is provided in the semiconductor substrate, andthe frame and the sheet member comprises an epitaxial layer provided onthe semiconductor substrate.
 2. The element according to claim 1 whichis used in the acceleration sensor for sensing the acceleration as aresistance change due to elastic deformation of at least two flexibleparts wherein each of said at least two flexible parts comprises atleast one piezoresistor, and the sheet member comprises wiringsconnected to the piezoresistors.
 3. The element according to claim 1which is used in the acceleration sensor for sensing an acceleration asan electrostatic capacitance change due to elastic deformation of atleast two flexible parts and which comprises at least one electrodewhich is located on a portion of the sheet member or weight which isdisplaced by the displacement of the weight relatively to the frame anda wiring connected to the electrode.
 4. A method for producing a flexuretransducer element used in an acceleration sensor which senses anacceleration applied thereto, in which method, the element comprises aflexure transducer element which is used as an acceleration sensor forsensing an acceleration applied thereto comprises (A) a frame having anupper surface and a lower surface, (B) a sheet member which has aplurality of flexible parts and a center part, each flexible partextending between at least a portion of an inner edge of the frame andthe center part and being integrally connected to them, (C) a weightwhich has a neck part integrally connected to the center part of thesheet member and which is hung from the sheet member through the neckpart, and (D) a support member which supports the lower surface of theframe and of which inward side surface of said support member faces to aside surface of the weight through a first space therebetween, a secondspace which is continuous with the first space is defined between eachflexible part of the sheet member and the weight, a third space isdefined between the frame and the sheet member and the weight, the frameand the sheet member are connected to each other and the sheet memberand the weight are connected to each other in such a manner that, whenthe acceleration is applied to the element, at least two flexible partsare elastically deformed so that the weight is displaced relatively tothe frame, the weight and the support member are formed of asemiconductor substrate having a first main surface and a second mainsurface which are opposing to each other, the second space is formed byremoving a sacrificial layer which is provided in the semiconductorsubstrate, and the frame and the sheet member comprises an epitaxiallayer provided on the semiconductor substrate, the method comprising:(1) forming in the first main surface of the semiconductor substrate forthe formation of the weight having the neck part and the support member,the sacrificial layer which extends outward from a portion of an outerperiphery of the center part of the first main surface which center partis to constitute the neck part, (2) the epitaxial layer is formed on thefirst main surface after (1), and (3) after (2), carrying out thefollowing (3-a) to (3-c): (3-a) removing a portion of the substrate fromthe second main surface of the substrate using etching so that the sidesurface of the weight and the support member are formed, the supportmember including the side surface opposing to the side surface of theweight through the first space, (3-b) forming the third space throughthe epitaxial layer by removing a portion thereof using etching so thatat least a portion of the rest of the epitaxial layer is formed into theframe and the sheet member including the center part and a plurality ofthe flexible parts which finally becomes able to elastically deform, and(3-c) removing the sacrificial layer through wet etching so that thesecond space and the neck part of the weight are formed, whereby theweight is formed, in any one of the following orders (i) to (iv): (i)(3-a)→(3-b)→(3-c), (ii) (3-a)→(3-c)→(3-b), (iii) (3-b)→(3-a)→(3-c), and(iv) (3-b)→(3-c)→(3-a).
 5. The method according to claim 4 wherein uponthe formation of the sacrificial layer on the substrate, an impurityconcentration of at least the epitaxial layer selected from thesubstrate and the epitaxial layer is made higher than a concentration ofthe impurity which forms the impurity layer and which is taken into theepitaxial layer due to auto-doping upon epitaxial growth.
 6. The methodaccording to claim 4 wherein the first space defined by the sidesurfaces of the support member and the weight is so tapered that adistance between the support member and the weight is reduced in twosteps along a direction from the bottom to the neck part of the weight,and the first space is composed of a first part near the bottom of theweight and a second part on the first part, and a first tapering angleformed by the side surfaces of the support member and the weight issmaller than a second tapering angle formed by the side surfaces of thesupport member.
 7. The method according to claim 6 wherein the firstpart is formed by mechanical grinding, sandblast or the RIE.
 8. Themethod according to claim 4 wherein in step (1), the sacrificial layeris so formed that it surrounds the center part of the first mainsurface.
 9. The method according to claim 4 wherein in step (1), thesacrificial layer is formed as a plurality of elongated layers whichsymmetrically extend from the center part of the first main surface. 10.The method according to claim 5 wherein the sacrificial layer is animpurity layer which includes an impurity of which conductivity type isopposed to that of the substrate at a higher impurity concentration thanthat of the substrate, or a porous silicon layer.
 11. The methodaccording to claim 10 wherein in the impurity layer of which impurityconcentration is higher than that of the substrate, an impurityconcentration of a surface portion of the impurity layer is relativelysmaller than that of the inside of the impurity layer.
 12. The methodaccording to claim 11 wherein the impurity concentration of the surfaceportion of the impurity layer is made not larger than 5×10¹⁹ cm⁻³. 13.The method according to claim 11 wherein the impurity layer is formed bydeposition and thermal diffusion of the impurity to the substratefollowed by wet oxidation or pyrogenic oxidation, so that the impurityconcentration of the surface portion of the impurity layer is relativelysmaller than that of the inside of the impurity layer.
 14. The methodaccording to claim 11 wherein the impurity layer is formed by directimplantation of an impurity ion to the substrate followed by anannealing treatment, so that the impurity concentration of the surfaceportion of the impurity layer is relatively smaller than that of theinside of the impurity layer.
 15. The method according to claim 11wherein after the formation of the impurity layer, another impurity ofwhich conductivity type is different is doped into a vicinity of thesurface portion of the impurity layer, so that the impurityconcentration of the surface portion of the impurity layer is relativelysmaller than that of the inside of the impurity layer.
 16. The methodaccording to claim 4 wherein (3) after (2) includes (3-d) which iscarried out before, after or between any two of (3-a) to (3-c) and inwhich at least one piezoresistor is formed on each of at least twoflexible parts, and wirings connected to the piezoresistors are formedon the sheet member.
 17. The method according to claim 16 wherein whenthe etching is carried out after (3-d), at least one protection layer isformed before the etching so that the piezoresistors and the wirings, orthe electrode and the wiring are protected.
 18. The method according toclaim 17 wherein the protection layer is a silicon nitride film or afluoroplastic film.
 19. The method according to claim 17 wherein thewiring further includes a pad electrode, and when the protection layerfurther protects the pad electrode, a thickness of that portion of theprotection layer which is located on the pad electrode is reducedbeforehand relatively to the other portion of the protection layerbefore the etching, and etching over an entire surface of the protectionlayer is carried out so that only the pad electrode is exposed.
 20. Themethod according to claim 4 wherein step (3) after step (2) includessub-step (3-e) which is carried out before, after or between any two ofsub-steps (3-a) to (3-c) and in which at least one electrode is formedon the sheet member which is displaced by the displacement of the weightrelative to the frame or on a portion of an upper surface of the weightand a wiring connected to the electrode is formed.
 21. The methodaccording to claim 20 wherein when the etching is carried out after(3-e), at least one protection layer is formed before the etching sothat the piezoresistors and the wirings, or the electrode and the wiringare protected.
 22. The method according to claim 5 wherein when sub-step(3-c) is carried out after sub-step (3-a), sub-step (3-a) is so carriedout that the formed first space reaches the sacrificial layer, andsub-step (3-c) is carried out with supplying an etchant through thefirst space.
 23. The method according to claim 4 wherein when sub-step(3-c) is carried out after sub-step (3-b), sub-step (3-b) is so carriedout that the formed third space reaches the sacrificial layer, andsub-step (3-c) is carried out with supplying an etchant through thethird space.
 24. The method according to claim 4 wherein the third spaceis so formed that it is located through a portion of the epitaxial layeron the sacrificial layer and/or adjacent to said portion, and the thirdspace leads to the sacrificial layer.
 25. The method according to claim4 wherein a cross sectional shape of the third space parallel to thefirst main surface is a circle, oval or rectangle having fourrounded-corners.
 26. The method according to claim 4 wherein whensub-step (3-c) is carried out after sub-step (3-b), sub-step (3-b) iscarried out in which the third space is so formed along the flexiblepart to be formed that the third space is located through a portion ofthe epitaxial layer on the sacrificial layer and/or adjacent to saidportion, and also that the third space reaches the sacrificial layer,and sub-step (3-c) is carried out with supplying an etchant through thethird space.
 27. The method according to claim 4 wherein at least oneflexible part is in the form of a beam which extends from the centerpart of the sheet member toward the frame, the third space comprises aspace which is surrounded by the frame and the at least one flexiblepart, and sub-step (3-c) is carried out with supplying an etchantthrough the third space.
 28. The method according to claim 4 wherein anopening of the third space through the epitaxial layer which opening isnot in contact with the substrate has a size based on such properties ofanisotropic etching that when the third space which is being formedreaches the sacrificial layer, step (3-b) is substantially stoppedautomatically.
 29. The method according to claim 4 wherein sub-steps(3-a) and (3-b) are carried out simultaneously.
 30. The method accordingto claim 4 wherein the third space is formed by the RIE.
 31. The methodaccording to claim 4 wherein a portion of the epitaxial layer throughwhich the third space is to be formed is formed into a high impurityconcentration portion, and sub-step (3-b) is carried out by removing theportion with etching.
 32. The method according to claim 20 whereinsub-steps (3-b) and (3-c) are carried out continuously.
 33. The methodaccording to claim 4 wherein when sub-step (3-c) is carried out aftercarrying out sub-steps (3-a) and (3-b), sub-step (3-c) is carried out bysupplying the etchant through the first space and the third space. 34.The method according to claim 4 wherein the etching of sub-step (3-a) isstopped before the first space reaches the sacrificial layer so that aportion of the substrate is left between the first space and thesacrificial layer, and after sub-step (3-c), the etching is carried outso that the left portion is removed, whereby the formed second spacereaches the first space.
 35. The method according to claim 34 whereinthe left portion is removed by anisotropic etching in which an etchantbased on an alkaline is used or by the RIE.
 36. The method according toclaim 4 which further comprises the step of etching a bottom surface ofthe weight so that a thickness of the weight is reduced.
 37. The methodaccording to claim 35 wherein etching of the bottom surface of theweight so as to reduce the thickness of the weight is carried outsimultaneously with the removal of the left portion.